Final answer:
Melatonin is the hormone that is thought to play a role in circadian rhythms. It is produced by the pineal gland and is involved in regulating sleep-wake cycles.
Step-by-step explanation:
The hormone that is thought to play a role in circadian rhythms is melatonin (option c). Melatonin is produced by the pineal gland and is involved in regulating sleep-wake cycles. Its release is stimulated by darkness and inhibited by light. When light levels decline, melatonin production increases, promoting drowsiness.