**Core Concept**
The P wave in an electrocardiogram (ECG) represents the depolarization of the atria, marking the initiation of the cardiac cycle. This depolarization is triggered by the electrical impulse generated by the sinoatrial (SA) node, which acts as the heart's natural pacemaker.

**Why the Correct Answer is Right**
The P wave is caused by the depolarization of the atria, specifically the right and left atria. This depolarization is mediated by the electrical impulse generated by the SA node, which is located in the right atrium. The impulse travels through the atrial tissue, causing the atria to contract. The depolarization of the atria is a result of the opening of voltage-gated sodium channels, which allow sodium ions to rush into the cardiac cells, causing a rapid depolarization.

**Why Each Wrong Option is Incorrect**
* **Option A:** Incorrect because the P wave is not caused by the depolarization of the ventricles, which is represented by the QRS complex in the ECG.
* **Option B:** Incorrect because the T wave is caused by the repolarization of the ventricles, not the depolarization of the atria.
* **Option C:** Incorrect because the ST segment represents the period of ventricular repolarization, not the depolarization of the atria.

**Clinical Pearl / High-Yield Fact**
The P wave is a critical component of the ECG, and its absence or alteration can indicate various cardiac conditions, such as atrial fibrillation or atrial flutter.

**Correct Answer: C. Depolarization of the atria.**