Final answer:
The Retino-Hypothalamic path regulates circadian rhythms via the neurotransmitter melatonin, which is produced in the pineal gland and is inhibited by light and stimulated by darkness.
Step-by-step explanation:
The Retino-Hypothalamic path is responsible for the regulation of circadian rhythms, and the primary neurotransmitter involved is melatonin. This path begins in the retina and ends in the suprachiasmatic nucleus (SCN) of the hypothalamus, which is known as the body's internal clock. The SCN receives information about the amount of light present, and this allows the internal clock to synchronize with the external day/night cycle. When light stimulates the retina, the SCN signals the pineal gland to inhibit melatonin production, leading to reduced drowsiness and increased wakefulness. Conversely, darkness triggers the release of melatonin, promoting sleep and adjusting the sleep-wake cycle. Therefore, the correct answer to the student's question is c) Melatonin.