**Core Concept**
The question is testing the understanding of the autonomic nervous system's (ANS) control over the heart, specifically the regulation of cardiac conduction. The ventricular muscle receives impulses directly from the Bundle of His, which is part of the electrical conduction system of the heart.

**Why the Correct Answer is Right**
The Bundle of His is a group of specialized cardiac muscle fibers that arises from the atrioventricular (AV) node and travels down the septal surface of the heart to the ventricles. It is responsible for conducting the electrical impulse from the AV node to the ventricular muscle, allowing for coordinated contraction of the ventricles. This is in contrast to the SA node, which initiates the heartbeat but does not directly stimulate the ventricular muscle.

**Why Each Wrong Option is Incorrect**
**Option A:** The SA node is the natural pacemaker of the heart, initiating the heartbeat by producing electrical impulses that travel through the atrial muscle to the AV node. However, it does not directly stimulate the ventricular muscle.
**Option B:** The AV node is a critical part of the electrical conduction system, but it only receives impulses from the SA node and then passes them on to the Bundle of His, which then stimulates the ventricular muscle.
**Option C:** The Purkinje fibers are a type of specialized cardiac muscle that arises from the Bundle of His and is responsible for rapidly conducting the electrical impulse to the ventricular muscle, but they are not the direct source of the impulse.

**Clinical Pearl / High-Yield Fact**
The Bundle of His is a critical structure in the electrical conduction system of the heart, and damage to it can lead to complete heart block, a condition where the ventricles do not contract in response to the electrical impulse from the AV node.

**Correct Answer:** D. The Bundle of His.