Final answer:
The aortic valve prevents the backflow of blood into the left ventricle when the ventricles relax, during the phase known as ventricular diastole.
Step-by-step explanation:
When the ventricles relax, the aortic valve prevents the backflow of blood into the left ventricle. This phase of the cardiac cycle is referred to as ventricular diastole. During this phase, pressure in the ventricles falls below that in the aorta and pulmonary trunk, causing blood to flow back towards the heart. At this point, the semilunar valves, which include the aortic valve for the left ventricle and the pulmonary valve for the right ventricle, close to avoid backflow into the ventricles, a process indicated as the isovolumic ventricular relaxation phase. The answer to the question is A - Aortic.