**Core Concept**
The rapid depolarization in cardiac muscle cells is a critical event in the cardiac action potential, initiated by the opening of specific voltage-gated ion channels. This process leads to a rapid influx of positively charged ions into the cell, causing the membrane potential to become more positive.

**Why the Correct Answer is Right**
The correct answer is related to the activation of the sodium-potassium pump (Na+/K+-ATPase) and the opening of voltage-gated sodium channels (Nav1.5). During phase 0 of the cardiac action potential, the rapid depolarization is primarily due to the influx of sodium ions (Na+) through the voltage-gated sodium channels. This influx of positively charged ions causes the membrane potential to become more positive, leading to rapid depolarization of the cardiac muscle cell.

**Why Each Wrong Option is Incorrect**
**Option A:** This is incorrect because the rapid depolarization in cardiac muscle cells is not primarily due to the opening of calcium channels, although calcium channels do play a role in the cardiac action potential, particularly during phase 2 (the plateau phase).

**Option B:** This is incorrect because the sodium-potassium pump (Na+/K+-ATPase) is involved in maintaining the resting membrane potential and helping to repolarize the cell after the action potential, but it is not directly responsible for the rapid depolarization.

**Option C:** This is incorrect because the rapid depolarization in cardiac muscle cells is not primarily due to the opening of chloride channels, which can actually help to stabilize the membrane potential.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the cardiac action potential is a complex process involving multiple ion channels and pumps, but the rapid depolarization is primarily due to the opening of voltage-gated sodium channels.

**Correct Answer:** C.