During ventricular diastole, a. blood flows into the aorta. b. the ventricles contract. c. the semilunar valves are closed. d. Both a and b are correct.

Let's analyze each statement given in the exercise. a. Blood flows into the aorta: During ventricular diastole, the ventricles are in the relaxation phase, and the blood flows into the ventricles from the atria, not into the aorta. b. The ventricles contract: The ventricular diastole is a period of relaxation, not contraction. Therefore, this statement is false. c. The semilunar valves are closed: During ventricular diastole, the ventricles are filling with blood, and this filling requires the semilunar valves (aortic and pulmonary valves) to be closed to prevent blood from flowing back into the ventricles from the aorta and pulmonary arteries. Thus, this statement is true. d. Both a and b are correct: As established in the earlier analysis, both statements a and b are false. Therefore, this statement is also false.

Based on the analysis of each statement, the correct answer is: c. the semilunar valves are closed.