• The smallest cell is Mycoplasma (0.1-0.5$\mu m$). 
• Bacterial cell measures 3-5$\mu m$ in length. 
• Erythrocytes are 6-8$\mu m$ in diameter. 
• The larges cell is the egg of Ostrich.

The body size of the organism is directly related to the cell number, not the cell size. The bigger the body, the more will the number of cells.

An organism which is made of more than one cell is called a multi-cellular organism. Humans, animals and plants are some multi-cellular organisms which are made of millions to trillions of cells.

As the cell becomes larger, the activity of DNA is increased since large number of activities take place in the same cell. The cell membrane increases its size because as the cell enlarges, more and more ions and gases and other molecules are exchanged between the cells and external environment. Therefore, the cells surface area has to increase faster than the volume of the cell or else the cell membrane could rupture.

A cell is defined by definitive structures of the cell membrane and cytoplasm and its capacity of performing the vital metabolic activities. The viruses do not have a defined cellular structure. They are made of a protein coat in which a genetic material (either DNA or RNA) is present. They are unable to perform their cellular activities. They are obligate parasites that depend on the machinery of the host cell to perform their multiplication as well as metabolism.

The proximal convoluted tubule(PCT) of the nephron in the kidney has microvilli. Thus, it has 2 mucosal linings whereas rest others have only one mucosal lining. So, the correct answer is 'PCT'.

Cells in your body get energy from the foods that are eaten, such as glucose (a sugar). Cellular respiration converts glucose and oxygen into water, carbon dioxide and usable form of energy called ATP (this stands for adenosine triphosphate).

Viruses are an exception to cell theory as it is an acellular organism that is living inside the host and nonliving outside the host. In viruses, only one nucleic acid (DNA or RNA) is present. Thus, a virus can be either a DNA virus or an RNA virus. Viruses infecting animals usually have DNA as genetic material. Viruses infecting plants usually have RNA as genetic material. All eukaryotic cells have both DNA and RNA in their cells.

Nucleus is double membrane covered protoplasmic body that contains hereditary information. A nucleus is absent in some mature cells like mammalian RBCs and sieve tube cells in plants. They are called as enucleate cells. In mammals, erythrocytes also lose all other cellular organelles such as their mitochondria, Golgi apparatus and endoplasmic reticulum. As a result of not containing mitochondria, these cells use none of the oxygen they transport; instead they produce energy by glycolysis of glucose and lactic acid fermentation of the resulting pyruvic acid.

All the multicellular organisms start their life with a single cell (zygote). Later, it is divided and differentiated into different cell types in multicellular organisms. Cells are capable of dividing to form their own kind of cells. Thus, all cells of the body come from the pre-existing cells. Thus, the correct answer is option C.

The average diameter of bacteria is 0.2 to 2 micrometer. 
Mitochondrion size is 0.5 to 10 micrometer. 
The average diameter of a mammalian cell is 20 micrometer. 
Virus diameter ranges from 20 nanometers to 250-400 nanometer. 1000 nanometer = 1 micrometer. Hence, the virus particle is the smallest of all.

Cells of unicellular organisms are usually spherical due to surface tension. 
Cells of the multicellular organism are usually polyhedral. Because polyhedral cells have more neighbors which means more intercellular exchange can occur. Also, the polyhedral group of cells resists stretching to a more extent. 

In unicellular, reproduction occurs asexually and two new identical daughter cells that are produced. All multicellular start their life from a single cell i.e., zygote which is a product of the fusion (fertilization) of usually two haploid gametes, a female and a male gamete. The zygote (diploid) divide to give rise huge variety of different types of cell, specialized for different functions which in turn forms multicellular organism.

Cell size in unicellular organisms is 0.5-10 micrometer whereas, in multicellular organisms, cell size is 10-100 micrometers.

In 1809, Lamarck concluded that all living beings are formed of cells. Living substance was first observed by Corti in 1772 and was named sarcode by Dujardin in 1835. Dutrochet believed in the individuality of cells, i.e., living organisms are made up of a number of individual cells.

A multicellular organism attains its multicellularity from a single cell. Single totipotent cell undergo number of mitotic and meiotic divisions to give rise to a multicellular structure where cells undergo differentiation to a particular function. Some of the cells are undifferentiated such as stem cells which can give rise to any type of cells of the body. Also multicellular organisms undergoes dedifferentiation process where a differentiated cell revert back to totipotent cell and can again differentiate to a particular type of cell witha particular function. So Multicellular organisms posses all these type of cells so Answer D is correct.

In short a gamete is an egg cell (female gamete) or a sperm (male gamete). This is an example of anisogamy or heterogamy, the condition in which females and males produce gametes of different sizes. This is the case in humans, the human ovum has approximately 100,000 times the volume of a single human sperm cell. 

Schleiden and Schwann presented a cell theory which proposed that each cell is itself is an organism which has all attributes of life. Each cell is living its individual life as well as the as a group of tissue which has become an integral part of plant. So Schleiden while observing plant cells discovered this double life of each cell. So option A is correct.

The average size of a human cell is about 100 μm in diameter. The smallest of which is the red blood cell, and it also has not nucleus.

An organism which performs and exhibits all functions of life via a single cell, it is an unicellular organism. Such as prokaryotes like bacteria, eukaryotes like yeast are unicellular organisms. They lack conglomerate of cells (tissue) or organ systems to perform specific functions. All the cellular functions for it to be living are performed by the individual cell.  So, the correct answer is 'Unicellular'.

Reproduction is the characteristic for living organisms i.e., both unicellular and multi cellular organisms can reproduce irrespective of their cellular level. But, only multi cellular organisms have specialized organs for digestion and respiration/breathing that is GIT (Gastro Intestinal Tract) and Respiratory tract respectively, unlike unicellular organisms who respire and engulf food by general body surface. Thus, digestive tract and respiratory tract are defining feature of multicellular organism (ruminant). 
So, the correct answer is 'Both A and C'.

Some living organisms are made up of once cell only, these are called unicellular. These organisms have a large surface area to volume ratio and rely on simple diffusion to meet their needs. An example of a unicellular animal is Amoeba. Amoeba feed on smaller organisms such as bacteria.

The organisms can be broadly classified into two groups, namely unicellular organisms and multicellular organisms depending upon the number of cells. Unicellular organisms are made up of only one single cell. Examples include Amoeba, Paramecium, yeast, bacteria. Multicellular organisms are those organisms that are made up of more than one cell. Examples include human beings, animals.

The cell is a basic biological unit of living organisms.

A. merkel cells, keratinocytes, melanocytes andLangerhans cells.

B.epithelial cells, mucous cells.

C. astrocytes, oligodendrocytes andependymal cells.

D. goblet cells, columnar epithelial cells.

Most of our cells are surrounded by air.

There are certain organs which are parts of more than one system and perform functions for both of them. For example, the pancreas produces hormones where they have an endocrine role ad also secrete digestive juices being a part of digestive system. Similarly, male urethra acts as a duct for both male sperms as well as urine, thereby acting as a part of both reproductive system and excretory system. Bone marrow is a part of skeletal; as well as the circulatory system as it also produces RBC's and WBC'S. 

In all mammals, the ovum is fertilized inside the female body. typically visible to the naked eye without the aid of a microscope.

Cells are grouped into tissues, tissues are grouped into organs, different organs are grouped into organ systems and many organ systems come together to form complex organisms like human beings.

Cells are autonomous which means they are self contained. They are able to carry out their own activities independently for their survival. They are able to synthesis protoplasmic components of a cell from non living matter such as simple organic compounds. They are able to divide into daughter cells as they are capable of producing energy, exchange of gases and replication of genetic material required for division. Every such cell has a limited life span as it grows, divide and then later die but the other cells surrounding that cell may be in a different state of growth. Hence, option D is correct.