The is a low-pitched sound caused by vibration of the atrioventricular valves and surrounding fluid as the valves close at the beginning of ventricular systole.

First heart sound

Explanation:

First heart sound is the sound produced by the heart heard only by the stethoscope.

The first heart sound is not easily heard as the frequency of the sound iv very low and thus is considered low pitched sound.

The first sound is produced by the closure of the mitral cuspid in the left heart when the left ventricle is full and the tricuspid blood when the right ventricle is filled with blood.

The closure of these valves is followed by the ventricular contraction therefore the closure of these valves is considered the initial stage of the systole.

Thus, first heard sound is the correct answer.