**Core Concept**
The opening of the aortic valve is a critical event in the cardiac cycle, marking the transition from the isovolumetric contraction phase to the ejection phase. This event is governed by the pressure gradient between the left ventricle and the aorta, as well as the mechanical properties of the aortic valve leaflets and the ventricular pressure.

**Why the Correct Answer is Right**
The aortic valve opens when the pressure in the left ventricle exceeds the pressure in the aorta, typically during the peak of ventricular systole. This is facilitated by the rapid increase in ventricular pressure, which overcomes the resistance offered by the aortic valve leaflets and allows blood to flow from the left ventricle into the aorta. The aortic valve is a bicuspid valve, with two leaflets that are hinged at the base and open towards the apex.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because the aortic valve opens during systole, not during diastole. During diastole, the aortic valve is closed, and the ventricle is filling with blood.
* **Option B:** This option is incorrect because the aortic valve opens at the peak of ventricular systole, not at the beginning of systole. The initial phase of systole is characterized by isovolumetric contraction, where the ventricle contracts without ejecting blood into the aorta.
* **Option C:** This option is incorrect because the aortic valve opens due to the pressure gradient between the left ventricle and the aorta, not due to the increase in ventricular volume. While ventricular volume does increase during systole, the primary mechanism driving aortic valve opening is the pressure gradient.

**Clinical Pearl / High-Yield Fact**
The aortic valve opens at a pressure of approximately 80-100 mmHg, which is the typical peak systolic pressure in the left ventricle. This is a critical value to remember, as it helps clinicians understand the normal functioning of the aortic valve and diagnose conditions such as aortic stenosis, where the valve is narrowed and cannot open properly.

**Correct Answer: C. During the peak of ventricular systole.**