First step is nucleophilic substitution of a geminal dihalide

Steps:

• Identify reactant: 1,1-dichloropropane (CH3CH2CHCl2) has two Cl atoms on the same carbon.
• Aqueous NaOH provides OH- nucleophile, which attacks the carbon bearing Cl.
• One Cl is replaced by OH, forming CH3CH2CH(OH)Cl (1-chloro-1-propanol).
• This substitution sets up hydrolysis to propanal in later steps.

Why D is correct:

• Substitution replaces one halogen with OH via SN1 or SN2 mechanism, where OH- displaces Cl- per the general reaction R-X + OH- → R-OH + X-.

Why the others are wrong:

• A. Addition involves electrophilic attack on unsaturated bonds like alkenes, not present in the saturated dihalide.
• B. Elimination removes HX to form double bonds (e.g., E2 mechanism), yielding alkenes, not aldehydes.
• C. Oxidation requires loss of electrons or addition of oxygen without halogen replacement, unlike the initial Cl-to-OH swap.

Final answer: D