## Core Concept
The cell cycle is divided into several phases, including G1 (gap 1), S (synthesis), G2 (gap 2), and M (mitosis). DNA synthesis, the process by which a cell makes an exact copy of its DNA before cell division, is a critical aspect of cell replication.

## Why the Correct Answer is Right
The correct answer, **S phase**, is the period during the cell cycle when DNA synthesis occurs. During this phase, the cell replicates its DNA, ensuring that each daughter cell will receive a complete and identical set of chromosomes. This process is crucial for cell division and genetic continuity. The S phase is part of the interphase, which also includes G1 and G2 phases.

## Why Each Wrong Option is Incorrect
* **Option A (G1 phase):** This is the first growth phase of the cell cycle, where the cell prepares for DNA replication by producing organelles and proteins. DNA synthesis does not occur here.
* **Option C (G2 phase):** This is the second growth phase, occurring after DNA synthesis and before mitosis. The cell continues to produce organelles and proteins and prepares for cell division. No DNA synthesis occurs in this phase.
* **Option D (M phase):** This is the mitosis phase, where the replicated DNA and cellular components are divided between two daughter cells. DNA synthesis does not occur here; rather, the replicated chromosomes are separated.

## 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 replication can lead to mutations.

## Correct Answer: B. S phase.