**Core Concept**
The question is testing the understanding of the components used in Polymerase Chain Reaction (PCR), a widely used molecular biology technique for amplifying specific DNA sequences. The correct answer is a cation that plays a crucial role in stabilizing the double-stranded DNA structure during the PCR process.

**Why the Correct Answer is Right**
Magnesium ions (Mg2+) are the primary cation used in PCR. They help stabilize the double-stranded DNA structure by binding to the phosphate groups, thereby reducing the melting temperature of the DNA. This allows the Taq polymerase enzyme to efficiently synthesize new DNA strands. The concentration of magnesium ions is critical, as excessive levels can inhibit the polymerase activity.

**Why Each Wrong Option is Incorrect**
* **Option B:** Calcium ions (Ca2+) are not typically used in PCR, as they can interfere with the polymerase activity and DNA stability.
* **Option C:** Potassium ions (K+) are not directly involved in the PCR process and are not a suitable substitute for magnesium ions.

**Clinical Pearl / High-Yield Fact**
Mg2+ is essential for maintaining the stability of the DNA double helix, and its optimal concentration is crucial for successful PCR amplification.

**Correct Answer: C. Magnesium ions (Mg2+)**