Light is an essential factor for photosynthesis. There is a linear relationship between incident light and $CO _{2}$ fixation at low light intensities. At higher light intensities, gradually the rate does not show further increase as other factors become limiting. Light between 400-700 nm wavelength constitute the photosynthetically active radiation (PAR). Maximum photosynthesis takes place in red and blue light of the visible spectrum and minimum photosynthesis takes place in the green light. The most effective wavelength of light for photosynthesis is red because maximum absorption by chlorophyll a takes place in the red light.

Growth is an irreversible increase in the size. There are three developmental stages of growth of all plant parts including shoot and root. These are cell division, cell enlargement and cell differentiation. Cell division is the first phase that results in increase in the number of cells. In plants, karyokinesis, division of nucleus is followed by cytokinesis, the division of the cytoplasm. Cell enlargement gives the proper size to the organ and tissue. During this, protoplasm, vacuole, etc are formed. Cell differentiation is seen at cell level, tissue level, organ level and at the level of an organism. The cells differentiate to form different types of tissues that later perform different but specific functions.

Water is essential in the following ways:-

Answer is option A i.e. "a and b only"
Light between 400-700 nm wavelength constitute the photosynthetically active radiation (PAR). Maximum absorption of light by chlorophyll a occurs in red and blue regions of the absorption spectrum. So that, the rate of photosynthesis is maximum in red and blue light of the visible spectrum.

Plant growth regulators also called as phytohormones are the plant hormones that help in the various processes of plant-like flowering, ripening of fruit etc. They are secreted in the root, shoot and include all part of the plant body and any tissue is able to release and any tissue is able to accept them and work accordingly. These are auxin, gibberellin, cytokinesis etc.

Answer is option C i.e. "Critical concentration"
The concentration of the essential element below which plant growth is retarded is termed as critical concentration. Below this minimum level, plants start to show nutrient deficiency.
While, optimum concentration is the most desirable or satisfactory level of a nutrient. Maximum concentration is the highest concentration of nutrient and nutrient toxicity occurs when excess consumption of a nutrient does harm to an organism.

Growth in plants is affected by a number of environmental factors. The most important ones are light and temperature. This is because plants are autotrophic organisms and use light energy to produce their food. Since plants are living organisms, their metabolism requires ambient temperature and pH. This is because all the metabolic reactions are catalyzed by enzymes.

The living differentiated cells that by now have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation. Example: formation of meristems - interfascicular cambium and cork cambium from fully differentiated parenchyma cells.

The first sign of geotropic response occurs at a short distance away from the apex except in monocots in which case it occurs at the nodes where meristems are formed. The bending of a plant organ in response to gravity is called as gravitropism or geotropism.  Geostimulation results in rapid and marked changes in the node-puvinus, internode and sheath. These changes involves water content, dry matter accumulation, endogenous gibberellic acids and GA4 metabolism.

Chlorophyll molecules are light trapping green pigments in plants. They are made up of a cyclic tetrapyrrole head and a long phytol tail attached to the head. One atom of magnesium is directly coordinated to the four pyrrole rings in the cyclic tetrapyrrole head. Thus, magnesium is an important structural component of chlorophyll molecules. The iron is an essential micronutrient for plants. It is required for synthesis of chlorophyll and has several other functions also. Plants also require an exposure to light in order to synthesize chlorophyll.  Apparently because in the absence of light chlorophylls are not required at all; trapping of light being there chief function. In absence of light plants will be thin and pale. Such plants are called etiolated plants.

• Differentiation occurs in both external and internal features. The formation of two polar axis, i.e., differentiation of roots and shoots is the best example of external differentiation. 
• This differentiation occurs very early during embryonic development. The other example is the differentiation of the vegetative and reproductive phases.

Cambium is the layer which forms Cork and acts as a secondary meristem from a layer of collenchyma or parenchyma immediately beneath the epidermis. The cells which produce from cork cambium will differentiate and will get mature to perform a specialized function. The living cells which lost the capacity to divide can regain the capacity of the division under certain conditions is called as 'Dedifferentiation.'

 Xylem cells are dead and elongated hollow cells consists of secondary cell wall contains lignin.

Light and temperature are important for the life of plants. This paves the way for the importance of photoperiodism and vernalization in flowering plants.

Plant organs can detect a change in their orientation with respect of gravity because atleast they have to be kept for a week. When it is kept for a week, slowly changes in orientation of gravity could be sensed. This sensation is shown by them as they posses several tropic movements. Geotropism makes the changes in the plant. Shoots are negatively geotropic and roots are positively geotropic.

If a plant is flashed with red light initially and then flashed with far red light, the effect of red light is reversed. It is due to the presence of phytochromes which are present in plants as the pigments for light absorption. It has two forms, Pr and Pfr. The range of absorption in wavelength is from 660 nm to 730 nm.

In Leucosolenia, Gemmule formation (also called  internal budding ) occurs to tide over seasonal drought  or other adverse environmental conditions .

Phytochrome is responsible for absorption of red and far red light it is involved in red-far red light interconversion. Phytochrome is a photoreceptor, a pigment that plants use to detect light. It is sensitive to light in the red and far red region of the visible spectrum.

Photoblastic seeds require wavelength in the range of 660nm for the germination of seeds. Out of the two forms of phytochrome, Pr form absorbs light of 660nm wavelngth. It stimulates germination.

Seed germination requires sufficient amount of P$ _pr$fr. P$ _r$ is synthesized by germinating seeds and is converted to P$ _pr$ when exposed to red light. However, when  seeds are exposed to far-red light, they will not germinate as P$ _r$ is converted to P$ _r$.

Rate of plant growth and development depend upon various factors including temperature surrounding the plant. Each plant has a specific temperature range for optimum growth. The temperature affects plant growth processes such as photosynthesis, respiration, transpiration, protein synthesis, etc. In general, temperature range for plant growth is 0$^o$C to 35$^o$C. However, optimal day and night temperature range varies for plant growth.

In chloroplasts, the main pigment present is chlorophyll which is green in color. Growth of plant is affected by the different wavelengths of light as pigments in chloroplasts absorb different wavelengths of light for photosynthesis. White light is a mixture of many wavelengths of colors. Chlorophyll will absorb energies from all visible light except green. Hence exposing white light to a green plants will result in the fastest rate of photosynthesis which in turn result in fastest growth. Hence, optimum growth occurs in white light followed by blue or red.

A carbon-to-nitrogen ratio is a ratio of carbon to the nitrogen in a plant. In general, a high C/N ratio produces a high acid content and low methane production. In plants young tissues have less C/N ratio. Plants with high C/N ratio have more mechanical tissues and larger root system.

Photosynthesis requires light between 400 and 700 nm. Blue light promotes vegetative growth. Red and far-red promotes stem elongation. Red and blue light promotes flowering. Photoperiodism is best seen in red light as chlorophyll strongly absorbs red light and carries out photosynthesis. Hence, red light stimulates plant growth.

Short day plants require the long duration of the dark period. If the dark period is interrupted with a brief exposure to red light (660 nm), the flowering will not take place. The inhibitory effect of red light on flowering during the critical dark period in short day plants can be overcome by a subsequent exposure to far-red rays. This is due to the fact that when exposed to far-red light, P$ _fr$ is converted to P$ _r$.

Differentiation is the process by which cells move from a less specialized (such as stem cells) form to a more specialized form having a specific function. Dedifferentiation is the process whereby completely differentiated cells regress to a simpler state (such as stem cells) and gain the capacity to divide again. The process by which a group of once specialized cells (de-differentiated) return to their original specialized state is known as redifferentiation. Regeneration is the process of renewal or restoration of growth to recreate damaged tissues, limbs etc. So, the correct answer is 'dedifferentiation'.

The cells derived from root apical and shoot apical meristems and cambium differentiate and mature to perform specific functions. This process leading to maturation of cells is termed as differentiation. During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm e.g., to form a tracheary element the cells would lose their protoplasm, they also develop strong, elastic, lignocellulosic secondary cell walls, to carry water to long distances even under extreme tension. So, the correct answer is 'All of the above'.

The living differentiated cells, that have lost the capacity to divide can regain the power of division under certain conditions, this phenomenon is termed as de-differentiation. E.g.,formation of meristerns-interfascicular cambium (Formed from medullary rays) and cork cambium (formed from outer layer of cortex) from fully differentiated parenchyma cells. While doing so, such meristems/tissues are able to divide and produce cells that once again lose the capacity to deride but mature to perform specific functions, i.e., get redifferentiated. E.g., cells of secondary xylem, secondary phloem, periderm. Thus rediffrentiation can be defined as maturation or differentiation of dedifferentiated tissues. So, the correct answer is 'Redifferentiation'.

The portion of pith taken as an explant comprises of parenchymatous cells (i.e., simple permanent tissue which have lost the capacity to divide). When such cells are cultured on solid culture media, the parenchymatous cells of pith become meristematic and start dividing resulting in a mass of undifferentiated cells called callus. This is an example of dedifferentiation. 

Interfascicular cambium is the cambium formed by dedifferentiation of cells forming the ground tissue (parenchyma) that lie in line to the vascular cambium, since these strips of cambium originate between two vascular bundles or connect two vascular cambia together they are referred to as the interfascicular cambia. They are responsible for producing medullary rays during secondary growth.

Redifferentiation is the formation of new or differentiated tissue from a already differentiated tissue. During this process cells of parenchymatous tissue undergo dedifferentiation to meristematic tissue. The cells cut off by the meristematic tissue based on the location and function get modified to respective tissues like secondary cortex, secondary xylem and secondary phloem in roots, monocot stem, cork cells etc

Phytochrome is blue colored pigment present in the plants. It is a photoreceptor chromoprotein. It is mainly found in flowering plants. These are responsible for flowering. It mainly absorbs red and far-red radiation in plants and induces flowering process.

Phytochromes are photoreceptor found in plants that detects light. The different phytochromes are involved in different biological responses to red light. P$ _r$ is inactive form whereas P$ _{fr}$ is active form of phytochrome. When exposed to red light,  P$ _r$ is converted to P$ _{fr}$. This active form then initiate many biological processes in plants.

Phytochromes are the hormones that are light dependent and are bluish green pigments that are involved in the developmental processes. They are light dependent and activates the inactivated form of phytochrome. They sometimes are useful in seed germination and cell elongation but stops the elongation of the stem when getting too exposed to far-red light and get inactivated.

In callus culture, cell division in explant (differentiated mass of mature cells) forms and callus. Callus is an irregular unorganized and undifferentiated mass of actively dividing cells. The process is called dedifferentiation.

In plants, apical meristems divide and differentiate to perform a specific function. This phenomenon is called as differentiation. Sometimes, the differentiated cells in plants regain the capacity to divide under certain conditions. This phenomenon is called as dedifferentiation. An interfascicular vascular cambium is an example of dedifferentiation. During dedifferentiation, certain cells lose the capacity to divide and becomes mature. This phenomenon is called as redifferentiation.

Plants require four factors for its growth like:-

1. Extrinsic factors are those factors like light, water, temperature, availability of gases is the abiotic or environmental factors that control the growth of a plant. These are not controlled by the plant.
   
2. Intrinsic factors like growth regulators are released by the plant and are in direct control under the plant. They regulate process like cell differentiation, cell division etc.
   

The phonomenon of generation of whole plant by single cell is totipotency.It is the characteristic of plants mostly. When a plant of defferentiated tissue is cultured in a nutrient medium. Cells that are not dividing or are at resting stage undergo some changes and attain dividibility and become meristematic again. The process of reversion or regaining of meristematic activity to form callus is known as dedifferentiation while redefferentiation is the process in which the cell gains its specialization again.

Vivipary means that seed remains attached to the plant even during germination. It is an undesirable character for annual crops as they are not able to store under normal condition for coming years.

The phenomenon of generation of whole plant by single cell is totipotency.

• Plant growth regulators function as chemical messengers for intercellular communication. There are currently five recognized groups of plant hormones: auxins, gibberellins, cytokinins, abscisic acid ($ABA$) and ethylene.
• Indole-3-acetic acid ($IAA$, $3-IAA$) is the most common, naturally occurring plant growth regulator of the auxin class. 
  
• $2IP$ (Isopentenyl adenine) is a naturally occurring cytokinin that regulates cell division, development, and nutrient processing in plants.
• Zinc ($Zn$) is not a plant growth regulator. It is one of the eight essential micronutrients. It is needed by plants in small amounts, but yet crucial to plant development.
• So, the correct answer is '$IAA, 2IP, Zn$'.

The cells derived from root apical and shoot-apical meristems and cambium differentiate and mature to perform specific functions. This act leading to maturation is termed as differentiation. During differentiation, cells undergo few to major structural changes both in their cell walls and protoplasm. The living differentiated cells, that by now have lost the capacity to divide can regain the capacity of division under certain conditions. This phenomenon is termed as dedifferentiation. For example, formation of meristems – interfascicular cambium and cork cambium from fully differentiated parenchyma cells. While doing so, such meristems / tissues are able to divide and produce cells that once again lose the capacity to divide but mature to perform specific functions, i.e., get redifferentiated.