**Core Concept**
The plateau phase of the cardiac action potential is a unique feature of cardiac muscle physiology, distinct from other excitable cells like skeletal muscle and neurons. It is characterized by a prolonged depolarization phase, which allows for the coordinated contraction of the heart.

**Why the Correct Answer is Right**
The plateau phase is primarily due to the opening of slow calcium channels (L-type calcium channels) in the cardiac muscle cell membrane. These channels allow a continuous influx of calcium ions (Ca++) into the cell, which in turn maintains the depolarization of the membrane. This influx of calcium ions is crucial for the contraction of cardiac muscle cells. The slow calcium channels are activated by the depolarization of the cell membrane and remain open for a prolonged period, thus contributing to the plateau phase.

**Why Each Wrong Option is Incorrect**
**Option A:** Opening of fast Na+ channels is responsible for the initial rapid depolarization (upstroke) of the cardiac action potential, not the plateau phase.

**Option B:** Opening of Ca++-Na+ channels is not a well-defined mechanism in cardiac muscle physiology, and it does not contribute to the plateau phase.

**Option D:** Opening of K+ channels would lead to repolarization of the cell membrane, not depolarization, and thus is not responsible for the plateau phase.

**Clinical Pearl / High-Yield Fact**
The plateau phase of the cardiac action potential is crucial for the contraction of cardiac muscle cells. Any abnormality in the function of slow calcium channels, such as in long QT syndrome, can lead to arrhythmias and other cardiac complications.

**✓ Correct Answer: C. Opening of Slow Ca++ channels**