Explanation:

- Circadian Rhythm:

- It refers to the natural, internal process that regulates the sleep-wake cycle and repeats roughly every 24 hours.

- Commonly known as the body clock.

- Influences sleep patterns, feeding behavior, hormone release, and other important bodily functions.

- Option 1: Movement of a merry-go-round

- Unrelated to circadian rhythm.

- A merry-go-round is a rotating amusement ride.

- Option 2: Heartbeat

- Not directly related to circadian rhythm.

- Heartbeat is governed by the heart's electrical system.

- Option 3: Body clock

- Directly linked to circadian rhythm.

- Reflects the biological processes in 24-hour cycles.

- Option 4: Movement of a pendulum

- Unrelated to circadian rhythm.

- A pendulum's motion is governed by gravity and inertia.