**Core Concept**
DNA replication is a crucial process in the cell cycle where the genetic material is duplicated, ensuring that each new cell receives an exact copy of the parent cell's DNA. This process is tightly regulated and occurs during a specific phase of the cell cycle.

**Why the Correct Answer is Right**
DNA replication occurs during the S phase of the cell cycle, which is the synthetic phase. During this phase, the genetic material is duplicated, and the DNA is synthesized. This process is mediated by an enzyme called DNA polymerase, which reads the template strands of DNA and matches the incoming nucleotides to the base pairing rules (A-T and G-C). The S phase is a critical step in the cell cycle, as it ensures that each new cell receives a complete and accurate copy of the parent cell's DNA.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is not relevant to the cell cycle or DNA replication.
* **Option B:** The G1 phase is the first growth phase of the cell cycle, where the cell grows and prepares for DNA replication. While it is a critical phase, it is not where DNA replication occurs.
* **Option C:** The G2 phase is the second growth phase of the cell cycle, where the cell prepares for cell division. While it is a critical phase, it is not where DNA replication occurs.

**Clinical Pearl / High-Yield Fact**
The S phase is the most vulnerable phase of the cell cycle, as any errors in DNA replication can lead to mutations and cancer. Therefore, it is essential to maintain the integrity of the DNA during this phase.

**Correct Answer:** C. S phase.