**Core Concept**
The pacemaker potential refers to the spontaneous depolarization of cardiac cells in the sinoatrial (SA) node, leading to the generation of an action potential. This process is crucial for maintaining a regular heartbeat.

**Why the Correct Answer is Right**
The pacemaker potential is primarily driven by a decrease in potassium (K+) permeability. During diastole, the SA node cells lose their negative resting membrane potential due to a decrease in the outward flow of potassium ions. This decrease in K+ permeability allows the membrane potential to slowly depolarize, creating a pacemaker potential. The slow influx of sodium (Na+) and calcium (Ca++) ions also contributes to this process, but the decrease in K+ permeability is the primary mechanism.

**Why Each Wrong Option is Incorrect**
* **Option A:** Fast Na+ channels are primarily responsible for the initial rapid depolarization phase of the action potential, not the pacemaker potential.
* **Option C:** Slow Ca++ channels are involved in the plateau phase of the action potential, not the pacemaker potential.
* **Option D:** Rapid repolarization is the opposite of the pacemaker potential, which is characterized by slow depolarization.

**Clinical Pearl / High-Yield Fact**
The pacemaker potential is influenced by autonomic nervous system inputs, particularly the sympathetic nervous system, which increases the heart rate by increasing the rate of depolarization in the SA node.

**✓ Correct Answer: B. Decrease in K+ permeability**