What occurs during diastole?

During diastole, the heart muscle relaxes, allowing the ventricles to fill with blood. This phase is critical for the heart's overall function, as adequate filling of the ventricles ensures that when they contract during the next phase, enough blood is ejected to supply the body's needs. The relaxation of the heart muscle creates a negative pressure within the ventricles, facilitating blood flow from the atria into the ventricles.

This process is vital to maintaining efficient circulation and ensuring that the body receives an adequate supply of oxygenated blood. Hence, option B correctly describes this important phase of the cardiac cycle.