Substitution reactions replace one atom or group with another while preserving the carbon skeleton.

Steps:

• Define substitution as a reaction where an atom (like H) is swapped for another (like halogen) in an organic molecule, producing HX.
• Analyze option A: CH₃Cl loses an H and gains a Cl from Cl₂, forming CH₂Cl₂ and HCl—clear replacement.
• Eliminate D: No replacement; it's an unbalanced decomposition.
• Confirm A fits, as B and C involve different contexts or mechanisms not matching the primary definition here.

Why A is correct:

• It follows the general formula R-H + X₂ → R-X + HX for free radical substitution, where H in CH₃Cl is directly replaced by Cl.

Why the others are wrong:

• B: Represents halogenation of an alkane but is classified as initiation of chain reaction, not pure substitution in this context.
• C: Involves nucleophilic attack on alcohol, classified as functional group interconversion rather than simple substitution.
• D: Shows decomposition into simpler molecules without any atomic replacement.

Final answer: A