Final answer:
The semilunar valves open during ventricular systole, allowing blood to be ejected from the ventricles into the aorta and the pulmonary artery.
Step-by-step explanation:
The semilunar valves open during ventricular systole, which is when the ventricles contract. This phase occurs after the atria relax (atrial diastole), and the increased pressure in the ventricles forces the semilunar valves open, allowing blood to be ejected from the ventricles into the aorta and the pulmonary artery. The correct answer to the question 'During which phase do the semilunar valves open, allowing blood to be ejected from the ventricles?' is A) Ventricular systole.This phase occurs when the ventricles contract, pushing blood out of the heart. The opening of the semilunar valves allows blood to flow into the pulmonary trunk and aorta.