Final answer:
The S2 heart sound is heard during the d) isovolumetric relaxation phase of the cardiac cycle when the semilunar valves close to prevent blood from flowing back into the heart.
Step-by-step explanation:
S2 heart sound is heard during the isovolumetric relaxation phase of the cardiac cycle.
In the isovolumetric relaxation phase, the ventricular muscle relaxes and pressure on the remaining blood within the ventricle begins to fall. During this phase, the semilunar valves close to prevent backflow into the heart. Since the atrioventricular valves remain closed, there is no change in the volume of blood in the ventricle.
Therefore, the S2 heart sound, also known as the second heart sound or 'dub,' is heard during the isovolumetric relaxation phase when the semilunar valves close to prevent blood from flowing back into the heart.