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The phase of the cardiac cycle that corresponds to cardiac output is systole. During systole, the heart muscle contracts, specifically the ventricles, which forces blood out of the heart and into the circulation. This contraction is crucial for generating the pressure needed to overcome systemic and pulmonary pressures, allowing blood to be ejected effectively.

Cardiac output, defined as the volume of blood the heart pumps per minute, is significantly influenced by the efficiency and strength of the heart's contraction during this phase. As the ventricles contract, the blood is propelled into the arteries, and the volume of blood pumped during each contraction (stroke volume) multiplied by the heart rate determines the total cardiac output.

In contrast, diastole refers to the relaxation phase when the heart fills with blood. Preload is related to the initial stretching of the cardiac muscle fibers prior to contraction and is a determinant of stroke volume but does not directly represent the phase of the cardiac cycle in which output occurs. Afterload is the resistance the heart must overcome to eject blood and significantly impacts ventricular performance but is also not about the phase of contraction itself.

Thus, systole is the correct answer as it directly involves the contraction of the heart and the resulting cardiac output.