**Core Concept**
Calcium channel blockers (CCBs) are a class of medications that act by inhibiting the influx of calcium ions (Ca2+) into cardiac and smooth muscle cells, thereby reducing the force of contraction and heart rate. The effect of CCBs on cardiac conduction is a critical aspect of their therapeutic and side effect profiles.

**Why the Correct Answer is Right**
Verapamil is a non-dihydropyridine CCB that primarily affects the L-type calcium channels in the heart. By blocking these channels, verapamil decreases the influx of calcium ions into the cardiac muscle cells, which in turn reduces the contraction force and slows the heart rate. This is particularly effective in the atrioventricular (AV) node, where verapamil has a potent effect on prolonging the AV nodal conduction time. This action is responsible for verapamil's efficacy in treating conditions such as supraventricular tachycardia and hypertension.

**Why Each Wrong Option is Incorrect**
* **Option A:** Diltiazem is another non-dihydropyridine CCB, but it has a more balanced effect on both the heart and vascular smooth muscle. While it does affect cardiac conduction, its effect is not as pronounced as verapamil's.
* **Option B:** Amlodipine is a dihydropyridine CCB that primarily affects vascular smooth muscle. It has a minimal effect on cardiac conduction and is primarily used to treat hypertension and angina.
* **Option C:** Nifedipine is also a dihydropyridine CCB that primarily affects vascular smooth muscle. Like amlodipine, it has a minimal effect on cardiac conduction and is primarily used to treat hypertension and angina.

**Clinical Pearl / High-Yield Fact**
Verapamil's effect on the AV node makes it a useful agent in the treatment of supraventricular tachycardia, but it can also lead to significant bradycardia and AV block, particularly in patients with pre-existing conduction system disease.

**Correct Answer:** C. Verapamil.