**Core Concept**
The first heart sound (S1) occurs due to the closure of the atrioventricular (AV) valves, specifically the mitral and tricuspid valves, marking the beginning of ventricular systole. This event is a result of the sudden contraction of the ventricles, which increases the pressure inside the ventricles and forces the AV valves to shut.

**Why the Correct Answer is Right**
As the ventricles contract, the pressure inside the ventricles increases rapidly, exceeding the pressure in the atria. This pressure gradient causes the AV valves to close, producing the first heart sound. The mitral valve, also known as the bicuspid valve, is responsible for separating the left atrium and left ventricle, while the tricuspid valve separates the right atrium and right ventricle. The closure of these valves is a result of the coordinated contraction of the atria and ventricles.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the first heart sound occurs at the start of ventricular systole, not at the end of ventricular diastole, when the ventricles are still relaxing.
* **Option B:** This option is incorrect because the second heart sound (S2) occurs at the end of ventricular systole, when the semilunar valves (aortic and pulmonary valves) close.
* **Option C:** This option is incorrect because the first heart sound is not associated with the closure of the semilunar valves but rather the AV valves.

**Clinical Pearl / High-Yield Fact**
The first heart sound is a crucial landmark in the cardiac cycle, marking the beginning of ventricular systole. It is often described as a "lub" sound, which is a result of the closure of the AV valves.

**Correct Answer:** C. At the start of ventricular systole