Final answer:
The pineal gland in the brain produces melatonin, a hormone that regulates the sleep-wake cycle, promoting sleep in low light conditions such as nighttime.
Step-by-step explanation:
The pineal gland is the endocrine structure in the brain that is responsible for producing the hormone melatonin, which promotes sleep. Located inferior and posterior to the thalamus, this gland responds to the light-dark cycle of the environment. With reduced light levels, such as during the night, the pineal gland's production of melatonin increases, leading to a rise in blood levels that induce drowsiness and help regulate the sleep-wake cycle. Conversely, during daylight, production of melatonin is minimized, promoting wakefulness.
Therefore, the answer to the student's question is: A. Pineal gland.