**Core Concept**
The question revolves around the phases of an action potential, specifically the overshoot phase, and how it affects membrane potential and ion flux. This requires understanding of **electrophysiology** and the role of **ion channels** in neuronal signaling.

**Why the Correct Answer is Right**
During the overshoot of an action potential, the membrane potential becomes more positive than the resting potential due to an influx of **sodium ions (Na+)**. This phase is characterized by the opening of **voltage-gated sodium channels**, allowing Na+ to rush into the cell. As a result, the **membrane potential** becomes less negative, moving towards a more positive value.

**Why Each Wrong Option is Incorrect**
**Option A:** This choice is incorrect because the influx of sodium ions actually increases during the overshoot.
**Option B:** Similarly, this option is wrong as the efflux of potassium ions is not the primary event during the overshoot phase.
**Option D:** This is also incorrect because the permeability to sodium increases, not decreases, during the overshoot.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that the overshoot phase is primarily due to the influx of **sodium ions**, and understanding this is crucial for grasping how **local anesthetics** work by blocking sodium channels, thus preventing action potentials.

**Correct Answer:** D. Potassium permeability