During ventricular diastole, the event that occurs is d) the ventricles are passively filling. This phase of the cardiac cycle is when the heart muscle relaxes, allowing the heart chambers to fill with blood.
Explanation: Ventricular diastole follows ventricular systole, where the ventricles contract to pump blood out of the heart. When the ventricles relax, the AV (atrioventricular) valves, which are the mitral and tricuspid valves, open. This allows blood to flow from the atria into the ventricles. As the ventricles fill passively, the SL (semilunar) valves remain closed. Therefore, the correct statement is that during ventricular diastole, the ventricles are passively filling with blood from the atria.