The answer is letter A.
Smooth muscles are muscles that perform involuntary contractions within the body and are mainly found in organs (e.g. intestines, liver). Heartbeats are not a function of smooth muscles, they are the function of cardiac muscles. Cardiac muscles refer to the muscles of the heart that play a vital role in pumping blood throughout our organ systems. Among all the muscle groups, the cardiac muscles are the ones exerting the most effort since the day that you were conceived up until the day that you die.