**Core Concept:** Ion channels play a crucial role in regulating the electrical activity of cells, including cardiac myocytes responsible for heart contractions. Ion repolarization is primarily governed by the potassium ion (K+) movement across the cell membrane.

**Why the Correct Answer is Right:** Potassium ions (K+) are primarily responsible for repolarization in the heart. Specifically, the repolarization process is facilitated by the activation of the rapid and slow delayed rectifier potassium channels. These channels help to return the membrane potential towards its resting state, thereby contributing to the normal cardiac action potential.

**Why Each Wrong Option is Incorrect:**

A. Calcium (Ca2+) is primarily responsible for depolarization in the heart, not repolarization. Calcium ions facilitate the opening of voltage-gated calcium channels, which leads to depolarization.

B. Sodium (Na+) is involved in depolarization but does not play a significant role in repolarization, as repolarization is primarily governed by potassium ions.

C. Hydrogen (H+) does not have a direct role in cardiac repolarization. While acid-base balance affects membrane potential indirectly, the primary ions involved in repolarization are potassium and calcium ions.

D. Magnesium (Mg2+) has a role in maintaining the resting membrane potential and normalizing calcium homeostasis. However, it does not directly contribute to repolarization via specific ion channels.

**Clinical Pearl:** Understanding ion regulation is crucial for understanding the cardiac action potential and maintaining normal cardiac function. Imbalances in ion concentrations can lead to arrhythmias and other cardiac disorders.

**Correct Answer:** D.