Hint: There are some valves that are present in the heart that prevent the backflow of the blood from ventricles to atria. These are present between the atria and ventricles and they are responsible for generation of heart sound.
The first heart sound is generated by the atrioventricular valves closing. The heartbeat results in generation of "lub-dub' and the first heart sound, or $\mathop S\nolimits_1 $, forms the "lub" of "lub-dub" and is made of $\mathop M\nolimits_1 $ (mitral valve closure) and $\mathop T\nolimits_1 $ (tricuspid valve closure). Normally $\mathop T\nolimits_1 $ is first and is followed by $\mathop M\nolimits_1 $.
It occurs due to the closure of the atrioventricular valves, i.e. tricuspid and mitral (bicuspid) valves, at the beginning of the ventricular contraction, or systole.
The second tone, that is the "dub" or $\mathop S\nolimits_1 $, is triggered and carried by the aortic and pulmonary valve closures, which mark the end of the systole.
And time lag between the first heart sound and the second sound defines systole (ventricular ejection) and the time period between the second and the next first sound defines diastole (ventricular filling).
There is also generation of the sound of a third and fourth heart, $\mathop S\nolimits_3 $ and $\mathop S\nolimits_3 $. They may take place in normal people or may be associated with pathological processes.
$\mathop S\nolimits_3 $ and $\mathop S\nolimits_4 $ are called gallops due to the reason of their cadence or rhythmic pacing. Gallops are defined as the sounds of low frequency, consistent with diastolic filling. $\mathop S\nolimits_4 $ may take place in patients with very high blood pressure.
Hence, the correct answer is option (C)
Note: The electrocardiogram (ECG) is defined as an electrical device that is used to measure the rate of heartbeat. The electrodes that will be kept on the chest of a person and the signals are observed as a wave in the computer.