**Core Concept:** The atrioventricular (AV) node is a crucial part of the heart's conduction system that controls the electrical impulse transmission between the atria and ventricles. The effective refractory period (ERP) refers to the duration during which the heart tissue is unable to generate another impulse after a previous one, preventing the heart from beating too quickly.

**Why the Correct Answer is Right:** **Option C, Calcium Channel Blockers (CCBs)** such as Verapamil and Diltiazem cause a decrease in heart rate and increase in AV nodal ERP. Calcium channel blockers work by blocking the influx of calcium ions into cardiac myocytes, thereby reducing the force of contraction and slowing down the heart rate. In the context of the AV node, this leads to an increase in the ERP, causing a slower heart rate.

**Why Each Wrong Option is Incorrect:**

A. **Digoxin:** Digoxin is a cardiac glycoside that increases the contractility of the heart and its rate. It does not affect the AV node ERP, making it an incorrect choice.

B. **Beta-blockers:** Beta-blockers, like ACE inhibitors, reduce heart rate and contractility but do not increase the AV nodal ERP. As a result, they are incorrect as well.

C. **Calcium Channel Blockers (CCBs):** As explained above, CCBs increase the AV nodal ERP and heart rate, making them the correct answer.

D. **Antiarrhythmics (Class II and III):** These medications are used to regulate irregular heartbeats (arrhythmias). They can affect the AV node and increase its ERP, making them relevant but incorrect in this context, as we are specifically looking for a drug that increases AV nodal ERP.

**Clinical Pearl:** During the treatment of supraventricular tachycardias (rapid heart rhythms originating in the atria or ventricles), antiarrhythmics (Class II and III) are often used to slow down the heart rate. However, in this MCQ, we are discussing a drug that directly affects the AV node and its ERP.

**Correct Answer:** **Option C, Calcium Channel Blockers (CCBs)** can increase the AV nodal ERP and heart rate, making them the correct answer within the context of this question. CCBs are not typically used for modulating AV nodal ERP, and their primary indication is for angina relief and blood pressure control.