**Core Concept**
Proofreading during DNA replication occurs in the S phase (Synthesis phase) of the cell cycle, where DNA polymerase enzymes detect and correct mismatched nucleotides to maintain genomic fidelity.

**Why the Correct Answer is Right**
During the S phase, DNA replication takes place, and DNA polymerase III (in prokaryotes) or DNA polymerase δ/ε (in eukaryotes) incorporates nucleotides with a built-in 3'→5' exonuclease activity. This proofreading function removes incorrectly paired nucleotides immediately after they are added, significantly reducing mutation rates. This process is essential for preserving genetic accuracy before cell division.

**Why Each Wrong Option is Incorrect**
Option A: G1 phase is a growth phase where the cell prepares for DNA replication but does not involve DNA synthesis or proofreading.
Option C: G2 phase follows DNA replication and involves preparation for mitosis, not replication or proofreading.
Option D: M phase (mitosis) involves cell division and chromosome segregation, not DNA replication or proofreading.

**Clinical Pearl / High-Yield Fact**
DNA polymerase’s proofreading is most active during the S phase; failure here increases mutation risk and is linked to cancer and genetic disorders. Always remember: **"Proofreading happens in S phase, not during division."**

✓ Correct Answer: B. S