## Core Concept
The cell cycle consists of several phases, including G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis). DNA synthesis is a critical process that occurs during one specific phase of the cell cycle.

## Why the Correct Answer is Right
The correct answer, **S phase**, is the phase during which DNA synthesis occurs. This is when the cell replicates its DNA, ensuring that each daughter cell will receive a complete set of chromosomes during cell division. The S phase is crucial for cell division and is tightly regulated to prevent errors in DNA replication.

## Why Each Wrong Option is Incorrect
* **Option A: G1 phase** - The G1 phase is the gap 1 phase, during which the cell grows and prepares for DNA replication. It does not involve DNA synthesis.
* **Option C: G2 phase** - The G2 phase is the gap 2 phase, which occurs after DNA synthesis and before mitosis. The cell continues to grow and prepare for cell division but does not undergo DNA synthesis.
* **Option D: M phase** - The M phase, or mitosis, is the phase where the cell divides into two daughter cells. DNA synthesis does not occur here; instead, the replicated DNA is divided between the two cells.

## Clinical Pearl / High-Yield Fact
A key point to remember is that the cell cycle is tightly regulated, and dysregulation can lead to cancer. The S phase is particularly critical because errors in DNA synthesis can lead to mutations.

## Correct Answer: B. S phase