**Core Concept**
During the depolarization phase of the action potential, the electrical properties of the neuronal membrane undergo significant changes, ultimately leading to the generation of an action potential. This phase is characterized by a rapid influx of positively charged ions, primarily sodium (Na+), into the cell.

**Why the Correct Answer is Right**
The depolarization phase is initiated by the opening of voltage-gated sodium (Na+) channels, allowing a rapid influx of Na+ ions into the cell. This influx of positively charged ions causes a rapid depolarization of the membrane potential. In contrast, potassium (K+) channels are typically closed during this phase, as the membrane potential is not yet at a level that would allow K+ ions to leave the cell in significant numbers. Therefore, K+ channels do not open during the depolarization phase, making option C the correct answer.

**Why Each Wrong Option is Incorrect**
**Option A:** Na+ channels do indeed open during the depolarization phase, allowing the rapid influx of Na+ ions into the cell.
**Option B:** Ca+ channels can also open during the depolarization phase, contributing to the generation of an action potential, particularly in neurons with a high density of Ca+ channels.
**Option D:** This option is incorrect because one of the options above (C) does not occur during the depolarization phase.

**Clinical Pearl / High-Yield Fact**
In cardiac muscle cells, the opening of Ca+ channels during the depolarization phase is crucial for the generation of the action potential and the initiation of muscle contraction. This is in contrast to skeletal muscle cells, where the action potential is generated solely by the opening of Na+ channels.

**✓ Correct Answer: C. K+ channels open**