Why is the repolarization of the atria not visible on an ECG?

The repolarization of the atria is not visible on an ECG predominantly because it is masked by the electrical activity of the ventricles. During the cardiac cycle, the ventricles generate substantial electrical signals as they depolarize and contract, which can obscure the smaller signals produced by the atrial repolarization. Specifically, the QRS complex, which represents ventricular depolarization, occurs after the atria have depolarized and before they repolarize, leading to an overlap in electrical activity. This overlap effectively hides the smaller repolarization signals of the atria.

In contrast, while repolarization does occur in the atria, it happens simultaneously with the depolarization of the ventricles, further complicating its detection on an ECG. The electrical activity during this phase is not significant enough to stand out against the prominent ventricular activity that is being recorded.

The other options do not accurately capture the reason: atrial repolarization indeed occurs, and it's not due to a weak signal or a distinct phase of the cardiac cycle. Therefore, the primary reason remains that the atrial repolarization is overshadowed by the stronger ventricular depolarization signals.