The correct answer is A. Atrial depolarization.
The P wave is the first wave of the electrocardiogram (ECG) and represents the depolarization of the atria. Depolarization is the electrical impulse that causes the muscle to contract. The P wave is normally upright in leads I, II, and aVF, and is inverted in lead aVR. The P wave is usually less than 0.12 seconds in duration and less than 2.5 mm in height.
The P wave is generated by the sinoatrial (SA) node, which is the natural pacemaker of the heart. The SA node is located in the right atrium and sends out an electrical impulse that causes the atria to contract. The P wave is followed by the QRS complex, which represents the depolarization of the ventricles. The QRS complex is usually wider than the P wave and is taller in amplitude. The QRS complex is followed by the T wave, which represents the repolarization of the ventricles. The T wave is usually upright in leads I, II, and aVF, and is inverted in lead aVR.
The P wave, QRS complex, and T wave are the three main waves of the ECG. The ECG is a valuable tool for diagnosing heart problems. It can be used to detect arrhythmias, heart block, and other conditions.