**Core Concept**
DNA synthesis, also known as the S phase, is a crucial period in the cell cycle where the genetic material is replicated. This process involves the unwinding of the double helix, the separation of the two strands, and the synthesis of new complementary strands by DNA polymerase enzymes.

**Why the Correct Answer is Right**
During the S phase, the cell's DNA is replicated through a series of complex biochemical reactions. The process begins with the unwinding of the double helix by helicase enzymes, which creates a replication fork. At the replication fork, the leading strand is synthesized continuously by DNA polymerase, while the lagging strand is synthesized discontinuously in short segments called Okazaki fragments. These fragments are later joined together by DNA ligase to form a continuous strand.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because the G1 phase is the first growth phase of the cell cycle, where the cell prepares for DNA replication by synthesizing proteins and organelles.
**Option B:** This option is incorrect because the G2 phase is the second growth phase of the cell cycle, where the cell prepares for mitosis by synthesizing organelles and proteins necessary for cell division.
**Option C:** This option is incorrect because the mitotic phase (M phase) is when the replicated DNA is divided between two daughter cells, not when DNA synthesis occurs.

**Clinical Pearl / High-Yield Fact**
The S phase is a critical period in the cell cycle, and any errors in DNA replication can lead to genetic mutations and cancer. The process of DNA replication is tightly regulated by multiple checkpoints to ensure accurate and efficient duplication of the genetic material.

**Correct Answer: S. The S phase of the cell cycle.**