Final answer:
The P wave on an ECG indicates atrial depolarization followed by atrial contraction, which is the correct answer, option A.
Step-by-step explanation:
The P wave of an ECG represents the depolarization of the atria, which is followed by atrial contraction. The correct answer to the student’s question is A: atrial contraction. This occurs after the P wave, when the atrial muscles contract and pump blood into the ventricles. The QRS complex follows and signifies the depolarization of the ventricles, leading to their contraction. Meanwhile, repolarization of the atria occurs during the QRS complex but is not visible on the ECG because it is masked by ventricular depolarization. The P wave is followed by the QRS complex, which represents the depolarization of the ventricles and is followed by ventricular contraction. Finally, the T wave indicates ventricular relaxation, signaling the repolarization of the ventricles and completing the cycle.