Final answer:
The T wave on an ECG represents the repolarization of ventricles after they contract, thus it appears positive on an ECG waveform. The atrial repolarization is hidden by the QRS complex.
Step-by-step explanation:
The T wave on an ECG waveform appears positive because it represents the repolarization of ventricles. This process occurs after the ventricles have contracted to pump blood to the lungs and body and must recover their resting state to be ready for the next contraction. While the atrial repolarization does occur, it is not represented as a separate wave on the ECG because it is masked by the much larger QRS complex which corresponds to ventricular depolarization. Therefore, the correct answer to the question is Option 2: Repolarization of ventricles.