**Core Concept**
Repolarization of a nerve refers to the process by which the nerve membrane returns to its resting state after an action potential. This process is crucial for the proper functioning of the nervous system, allowing neurons to transmit signals efficiently.

**Why the Correct Answer is Right**
Repolarization is primarily due to the flow of potassium ions (K+) out of the neuron, which is facilitated by the opening of potassium channels. As the potassium ions flow out, the membrane potential becomes more negative, returning the neuron to its resting state. The sodium-potassium pump also plays a role in maintaining the resting membrane potential by actively pumping sodium ions out of the cell and potassium ions into the cell.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because repolarization is not primarily due to the closure of sodium channels. While the closure of sodium channels does contribute to repolarization, it is not the primary mechanism.

**Option B:** This option is incorrect because chloride ions do not play a significant role in repolarization. The movement of chloride ions can affect the membrane potential, but it is not a key factor in repolarization.

**Option C:** This option is incorrect because calcium ions do not directly contribute to repolarization. Calcium ions are involved in other cellular processes, such as neurotransmitter release and muscle contraction.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that repolarization is a critical step in the action potential, allowing neurons to recover and prepare for the next signal transmission. Understanding the mechanisms of repolarization is crucial for grasping the complex physiology of the nervous system.

**Correct Answer:** K+ (Potassium ions)