Answer: A. A frog's heartbeat.
Step-by-step explanation:
Looking at the answer choices, all of the others are organisms doing actions willingly on their own accord (such as walking), the outlier in all of the answer choices would be A, a frog's heartbeat, since it's not like the frog can stop it's heart from beating anytime it wants; which is why it's an example of an organism using involuntary muscles.