**Core Concept**
The question is testing the student's understanding of the cell cycle, specifically the phase where DNA synthesis occurs. DNA replication is a crucial process that ensures the genetic material is duplicated and passed on to daughter cells during cell division.

**Why the Correct Answer is Right**
During the S phase (Synthesis phase), the cell's genetic material is replicated, and new DNA is synthesized. This process involves the unwinding of DNA double helices, the unwinding of the double helix, and the synthesis of new DNA strands by the enzyme DNA polymerase. The replication process is semi-conservative, meaning that one new strand is synthesized from each of the existing strands. The S phase is a critical phase of the cell cycle, and its proper execution is essential for the accurate transmission of genetic information.

**Why Each Wrong Option is Incorrect**
* **Option A:** The G1 phase (Gap 1 phase) is the first growth phase of the cell cycle, where the cell grows and prepares for DNA replication. It is not the phase where DNA synthesis occurs.
* **Option B:** The G2 phase (Gap 2 phase) is the second growth phase of the cell cycle, where the cell continues to grow and prepares for cell division. While some DNA repair and replication may occur in this phase, it is not the primary phase of DNA synthesis.
* **Option C:** The M phase (Mitosis phase) is the phase where cell division occurs, and the replicated DNA is divided between two daughter cells. This phase is not where DNA synthesis occurs.

**Clinical Pearl / High-Yield Fact**
The S phase is the most vulnerable phase of the cell cycle, as it is the phase where DNA is most susceptible to damage. This is why many chemotherapeutic agents target cells during the S phase, as they can exploit the cells' increased reliance on DNA synthesis.

**Correct Answer: C. S phase. The S phase (Synthesis phase) is the phase of the cell cycle where DNA synthesis occurs.**