Answer:
The correct option is d. All of the above contribute to the pinal gland being called the body's timekeeper.
Explanation:
The pineal gland is a reddish-grey pine-cone shape gland present in the brain. It is responsible for the production of melatonin. The melatonin is a hormone which controls the body's sleep cycle and the circadian rhythm.
The melatonin production is stimulated by darkness. Therefore, the melatonin levels are low during the days and increases at night. This helps in regulating the body's sleep cycle and the circadian rhythm.
Thus, the pineal gland is referred to as the body's timekeeper.