• Mechanisms of DNA Repair:
• Base Excision Repair (BER) & Nucleotide Excision Repair (NER): Remove damaged bases/adducts.
• Mismatch Repair (MMR): Corrects post-replicative base mispairings and insertion/deletion loops.
• Non-Homologous End-Joining (NHEJ): Directly ligates double-strand breaks without a homologous template.
• Branch Migration: A physical thermodynamic process where homologous strands exchange pairing partners during meiotic or mitotic homologous recombination (Holliday junction movement); it is NOT a DNA repair system.
• nalysis:
• (Branch migration): Correct (not a repair system).

Correct option: (4) Branch migration

• Alkylating Agents: Ethylmethanesulfonate (EMS) is a powerful chemical mutagen that transfers ethyl alkyl groups to nitrogenous bases in DNA molecules.
• Mechanism of Mutagenesis: EMS primarily ethylates the oxygen at position 6 of guanine, forming $O^6$-ethylguanine. During DNA replication, $O^6$-ethylguanine pairs with thymine instead of cytosine, producing characteristic $\text{G:C} \to \text{A:T}$ transition mutations.
• Other Mutagens: Ethidium bromide is an intercalating agent, nitrous acid is a deaminating agent, and 5-bromouracil is a thymine base analog.

Correct option: (2) Ethylmethane sulfonate.