Nucleophilic substitution with cyanide for chain lengthening Steps:

• Identify reaction as SN2 on primary alkyl halide requiring CN⁻ nucleophile.
• Select reagent providing high CN⁻ concentration: KCN over HCN (weak acid, low dissociation).
• Choose solvent: polar protic like alcohol for SN2; avoid acids that protonate CN⁻ to HCN.
• Apply heat under reflux to ensure completion without loss of volatile products.

Why A is correct:

• KCN dissociates fully to CN⁻; dilute alcohol (ethanol) is polar protic solvent promoting SN2 per standard organic mechanisms.

Why the others are wrong:

• B: Dilute sulfuric acid protonates CN⁻ (CN⁻ + H⁺ → HCN), lowering nucleophile availability.
• C: HCN gives low CN⁻ concentration; sulfuric acid further suppresses ionization.
• D: HCN in ethanol maintains low CN⁻ levels, insufficient for efficient substitution.

Final answer: A