**Core Concept**
During ventricular diastole, the ventricles relax and undergo a rapid filling phase, allowing the ventricular chambers to fill with blood from the atria. This phase is essential for maintaining cardiac output and ensuring adequate blood supply to the body's tissues.

**Why the Correct Answer is Right**
The correct answer is related to the opening of the atrioventricular (AV) valves, specifically the mitral and tricuspid valves. As the ventricles relax, the pressure in the atria decreases, causing the AV valves to open, allowing blood to flow from the atria into the ventricles. This process is facilitated by the decrease in ventricular pressure and the increase in atrial pressure.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it does not accurately describe an event that occurs during ventricular diastole. The ventricles are not contracting or ejecting blood during this phase.
* **Option B:** This option is incorrect because the ventricular systole is the phase of contraction, not relaxation. The ventricles are actively pumping blood out of the heart during systole.
* **Option D:** This option is incorrect because the opening of the semilunar valves (aortic and pulmonary valves) occurs during ventricular systole, not diastole.

**Clinical Pearl / High-Yield Fact**
The rapid filling phase of ventricular diastole is critical for maintaining cardiac output, especially during exercise or periods of increased demand. It is essential for clinicians to understand the normal physiology of cardiac function to diagnose and manage various cardiovascular disorders.

**Correct Answer: C. The atrioventricular (AV) valves open.**