Final answer:
The P wave of the ECG represents atrial depolarization. It signals the depolarization and contraction of the atria as they pump blood into the ventricles.
Step-by-step explanation:
The P wave of the ECG represents atrial depolarization. It is generated by the depolarization and contraction of the atria as they pump blood into the ventricles. The atria begin contracting shortly after the start of the P wave.
The P wave is followed by atrial contraction (systole). The QRS complex represents depolarization of the ventricles and is followed by ventricular contraction. Finally, the T wave represents the repolarization of the ventricles and marks the beginning of ventricular relaxation.
The repolarization of the atria occurs during the QRS complex, which masks it on an ECG. So, the correct answer is option 2) atrial depolarization.