Which of the following is a circadian rhythm?

• A. The molting of a crab
• B. The annual shedding of leaves by deciduous trees
• C. The human sleep-wake cycle
• D. The migration of monarch butterflies

Answer: (C)

Verified step by step guidance

1. Understand the concept of a circadian rhythm: A circadian rhythm is a biological process that follows a roughly 24-hour cycle, responding primarily to light and darkness in an organism's environment. It is regulated by an internal biological clock.
2. Analyze the options provided: Each option represents a biological process, but only one aligns with the definition of a circadian rhythm.
3. Option 1: The molting of a crab - This is a developmental process that occurs periodically but is not tied to a 24-hour cycle. It is regulated by growth and environmental factors, not a circadian clock.
4. Option 2: The annual shedding of leaves by deciduous trees - This is a seasonal process that occurs once a year, influenced by changes in temperature and daylight length, not a daily rhythm.
5. Option 3: The human sleep-wake cycle - This process follows a 24-hour cycle and is regulated by the circadian clock, making it a circadian rhythm. Option 4: The migration of monarch butterflies - This is a seasonal behavior, not a daily rhythm, so it is not a circadian rhythm.