Final answer:
The QRS complex on an ECG corresponds to the period of cardiac systole, where the ventricles contract and pump blood.
Step-by-step explanation:
The portion of the cardiac cycle on an ECG that corresponds to the period of cardiac systole is represented by the QRS complex. The QRS complex indicates ventricular depolarization, which is followed by ventricular contraction. Hence, when looking at an ECG, the QRS complex signals the start of systole where the ventricles contract and pump blood to the lungs and body. The P wave represents atrial depolarization and is followed by atrial contraction but is not the answer to cardiac systole. The T wave signifies the repolarization of the ventricles, indicating the beginning of ventricular relaxation, or diastole.