**Core Concept**
The QRS complex is a critical component of the electrocardiogram (ECG), representing the depolarization of the ventricles in the heart. This complex is generated by the rapid spread of electrical impulses through the ventricular muscle cells, causing them to contract and pump blood out of the heart.

**Why the Correct Answer is Right**
The QRS complex is a result of the depolarization of the ventricular muscle cells, which is triggered by the activation of the ventricular conduction system. This process involves the rapid influx of sodium ions into the ventricular muscle cells, leading to a rapid increase in the action potential and eventual contraction of the ventricles. The QRS complex is a representation of this rapid depolarization on the ECG.

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

**Clinical Pearl / High-Yield Fact**
It's essential to note that the duration of the QRS complex can provide valuable information about the functioning of the ventricular conduction system. A prolonged QRS complex (greater than 120 milliseconds) can indicate ventricular conduction delay or block.

**Correct Answer: B. Depolarization of the ventricles.**