Which of the following is used for the assessment of T-wave on an EKG?

The correct choice reflects an understanding of the cardiac cycle and the specific timing associated with the T-wave on an electrocardiogram (EKG). The T-wave represents ventricular repolarization, which occurs during the diastolic phase of the cardiac cycle. During diastole, the ventricles relax and refill with blood, and the electrical activity reflected by the T-wave corresponds to the recovery of the ventricles after contraction.

In contrast, during atrial systole and ventricular systole, the heart is either contracting (systole) or actively pumping blood, making it less relevant for assessing the status of the T-wave. Heart rate is an important aspect of overall cardiac function but does not specifically pertain to the T-wave itself. Thus, diastole is the correct phase of the cardiac cycle for assessing the T-wave on an EKG.