The pineal gland is a small endocrine gland in the vertebrate brain. It produces melatonin, a serotonin derived hormone, that affects the modulation of sleep patterns in both seasonal and circadian rhythms.

The diencephalon is the region of the embryonic vertebrate neural tube that gives rise to posterior forebrain structures including the thalamus, hypothalamus, posterior portion of the pituitary gland, and pineal gland. The pineal gland arises as a dorsal outgrowth of the caudal portion of the diencephalon. In the adult animal, it lies in the midline of the roof of the third ventricle. 

Serotonin is the hormone that contributes the feeling of well being and happiness in animals including humans. It is basically derived from tryptophan and is located in the gastrointestinal tract (GI tract), blood platelets and central nervous system. 90% of the total serotonin is primarily located in GI tract in Enterochromaffin cells and used for regulation of intestinal movements. Hence, answer D is correct.

Pinealocytes are the main cells contained in the pineal gland, located behind the third ventricle and between the two hemispheres of the brain. The primary function of the pinealocytes is the secretion of the hormone melatonin, important in the regulation of circadian rhythms.

The pineal gland or epiphysis synthesizes and secretes melatonin hormone, that communicates information about environmental lighting to various parts of the body. It is attached to the roof of third ventricle in the rear part of brain. It functions as a biological clock and a neurosensory transducer, converting the neural information.  

• Melanin is a pigment produced within the skin in melanocytes or skin cells when they are exposed to the sun. 
  
• The process by which the pigment melanin is formed by the melanocytes is called melanogenesis.
• The colour of the skin is determined by at least 6 genes.
• Thus, skin colour or complexion is controlled by the environment and is also hereditary.
• So. the correct answer is 'Both heredity and environment'.

• The pineal gland or pineal body (epiphysis or third eye) is a small endocrine gland.
  
• It secretes melatonin (hormone) which regulates the wake/sleep patterns and photoperiodic functions.
  
• Biorhythms or biological rhythms control sleep and wake cycles and other functions such as behaviour and endocrine rhythms. These functions are regulated by the circadian clock.
• The secretion of melatonin is triggered in dark and inhibited by light.
• So, the correct answer is 'Melatonin'.

• Photoperiodism is the physiological reaction to the length of day or night by an organism.
  
• The pineal gland or pineal body (epiphysis or third eye) is a small endocrine gland.
• It secretes melatonin (hormone) which regulates the wake/sleep patterns and photoperiodic functions.
  
• So, the correct answer is 'Pineal body'.

• Melanin is a pigment produced within the skin in melanocytes or skin cells when they are exposed to the sun. 
• The process by which the pigment melanin is formed by the melanocytes is called melanogenesis.
• The pigment is formed within a small membrane-bound structure called melanosomes.
  
• The skin colour is determined by a bluish-white connective tissue under the dermis or corium and the haemoglobin circulating in the veins of the corium.
  
• The corium is a layer of skin present between the epidermis and the subcutaneous tissues of the skin.
  
• Two types of melanin are formed in the body pheomelanin (red to yellow in colour) and eumelanin (dark brown to black).
  
• Those people with light complexion produce pheomelanin whereas, those with dark complexion produce eumelanin.
• So, the correct answer is 'Melanocytes of corium and incorporated in epidermis'.

• regulation of BMR (Basal metabolic rate) by thyroxine which increases consumption of glucose by metabolically active tissues.
• regulation of proteins, carbohydrates and fats metabolism.
• regulation of water and electrolyte balance by calcitonin which inhibits reabsorption of calcium by renal tubules.

The correct statement is- 'The pineal gland secretes a hormone melatonin which helps to regulate sleep pattern. Parathyroid Gland release hormone which regulates the level of calcium in the blood with the help of calcitonin which is produced in the thyroid gland.'

Pineal body is a very small, reddish-grey, vascular, conical, solid body located in the brain. It secretes a hormone named melatonin. Melatonin concentrates pigment granules in the melanocytes, making the skin colour lighter in certain animals. It also regulates the functioning of gonads, thus, controls, sexual behaviour and regulates the period of puberty.

Melatonin is a hormone secreted by pineal gland. Immediately above the optic chiasma(in the brain) in a nucleus, are present melatonin receptors that react to this hormone and synchronise the nucleus to the $24$hrs day/night rhythm, thus informing the brain when it is day and when it is night.

Melatonin, is a hormone that is produced by the pineal gland and regulates sleep and wakefulness. Blood levels of the pineal hormone melatonin are high at night and low during the day. There, a special center called the suprachiasmatic nucleus (SCN) initiates signals to other parts of the brain that control hormones, body temperature and other functions that play a role in making us feel sleepy or wide awake. During the day the pineal is inactive. When the sun goes down and darkness occurs, the pineal is "turned on" by the SCN and begins to actively produce melatonin, which is released into the blood. As a result, melatonin levels in the blood rise sharply and you begin to feel less alert. Sleep becomes more inviting. 

• Melatonin is a hormone that regulates sleep-wake cycles. It is primarily released by the pineal gland. 
• As a supplement, it is often used for the short-term treatment of trouble sleeping such as from jet lag or shift work. 
• Melatonin is a hormone which is a protein made up of amino acids. 
• Hence, Melatonin is a modified Amino acid. 
• So, the correct answer is 'amino acid'.

• The pineal gland is an endocrine gland located on the dorsal side of the forebrain. Pineal secretes a hormone called melatonin.
  
• Melatonin plays a very important role in the regulation of a 24-hour (diurnal) rhythm of our body. For example, it helps in maintaining the normal rhythms of the sleep-wake cycle, body temperature. In addition, melatonin also influences metabolism, pigmentation, the menstrual cycle as well as our defense capability. Hence the gland in our body that controls biological clock is a Pineal gland.

The circadian rhythm or 24 hours diurnal rhythm of our body is maintained by Melatonin hormone secreted from the Pineal gland which is located on the dorsal side of forebrain. In addition, the melatonin hormone even assist in metabolism, pigmentation and even maintaining our defence capabilities.