**Core Concept**
The QRS complex represents the depolarization of the ventricles in the electrocardiogram (ECG). It is a critical phase of the cardiac cycle where the electrical impulse spreads through the ventricular muscle, leading to muscle contraction.

**Why the Correct Answer is Right**
The QRS complex is a result of the depolarization of the ventricular muscle cells, specifically the rapid upstroke of the action potential. This is mediated by the opening of voltage-gated sodium channels in the ventricular muscle cell membrane, allowing an influx of sodium ions and a rapid depolarization of the cell. The depolarization wavefront then spreads through the ventricular muscle, leading to muscle contraction and the QRS complex on the ECG.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the P wave represents atrial depolarization, not ventricular depolarization.
**Option B:** This option is incorrect because the T wave represents ventricular repolarization, not depolarization.
**Option C:** This option is incorrect because the PR interval represents the time from the onset of atrial depolarization to the onset of ventricular depolarization, not the depolarization itself.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the QRS complex duration is a critical parameter in assessing ventricular conduction, and prolongation of the QRS complex (>120ms) can indicate ventricular conduction delay or bundle branch block.

**Correct Answer:** C. Depolarization of the ventricular muscle cells.