Ventricular muscle depolarization is indicated by

The correct option is The QRS complex.

An electrocardiogram (ECG or EKG) is a medical test that records the electrical activity of the heart. The resulting wave tracing consists of several distinct components, each representing a specific electrical event during the cardiac cycle:
1. P wave: Represents the depolarization of the atrial muscle, which triggers the contraction of the atria.
2. PR interval: Represents the time delay required for the electrical impulse to travel from the SA node, through the atria and AV node, and down to the ventricles.
3. QRS complex: Indicates the rapid depolarization of the ventricular muscle. Due to the large muscle mass of the ventricles compared to the atria, this complex is the most prominent waveform on a standard ECG.
4. U wave: A small wave occasionally observed after the T wave, generally representing the late repolarization of the ventricles (such as the Purkinje fibers).

Consequently, ventricular muscle depolarization is represented by the QRS complex.