Final answer:
The ventricular muscle is in a relaxed state during ventricular diastole, when the heart muscle as a whole is relaxed, during atrial systole, and for approximately half of the cardiac cycle, which matches all the provided options.
Step-by-step explanation:
Ventricular muscle is relaxed during ventricular diastole, which is one part of the cardiac cycle. Specifically, ventricular diastole occurs when the heart muscle is relaxed and blood flows into the heart.
Cardiac diastole is the period during which all the heart's chambers are relaxed, and this is when the ventricles are filling with blood. It is not just the ventricular muscle that is relaxed during this period but rather the entire heart. The atrial systole happens after the cardiac diastole and this is when the atria contract to push blood into the ventricles, but during atrial systole, the ventricular muscles are still relaxed. Thus, ventricular muscles are relaxed during ventricular diastole, atrial systole, and for approximately half of the cardiac cycle.