**Core Concept**
During the cell cycle, DNA replication occurs in the S phase, where the genetic material is duplicated to prepare for cell division. This process ensures that each daughter cell receives a complete set of chromosomes.

**Why the Correct Answer is Right**
The correct answer involves the concept of DNA replication and the cell cycle. DNA replication is a semi-conservative process where the genetic material is duplicated from a template strand. This process is catalyzed by enzymes such as DNA polymerase, which reads the template strand and adds nucleotides to the growing DNA strand. The result is two identical copies of the DNA molecule, each containing one old strand (the template) and one new strand (the replica). This process is essential for cell division, as it ensures that each daughter cell receives a complete set of chromosomes.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because DNA replication occurs in the S phase, not in the G1 phase, where cells prepare for DNA replication by synthesizing proteins and other essential molecules.
* **Option B:** This option is incorrect because DNA replication is not a function of the G2 phase, which is primarily involved in cell growth and preparation for cell division.
* **Option C:** This option is incorrect because DNA replication is not a function of the M phase, which is primarily involved in cell division.

**Clinical Pearl / High-Yield Fact**
A key point to remember is that DNA replication occurs in the S phase of the cell cycle, and it is essential for cell growth and division. This process is critical for the development and maintenance of tissues and organs.

**Correct Answer:** S. The S phase.