Final answer:
Ventricular filling begins during diastole phase of the cardiac cycle.
Step-by-step explanation:
Ventricular filling begins during diastole phase of the cardiac cycle. Diastole is the period of relaxation when the chambers of the heart fill with blood. During diastole, both the atria and ventricles undergo relaxation, allowing blood to flow into the ventricles. This filling stage is essential for efficient pumping of blood into circulation.