Nucleophilic addition involves a nucleophile attacking an electron-deficient center like a carbonyl carbon.

Steps:

• Recall that nucleophilic addition features a nucleophile adding to a π-bond or electrophilic center without substitution.
• Examine option A: alkene + HBr is electrophilic addition via carbocation intermediate.
• Examine option B: alkyl halide + NaOH in ethanol favors elimination (E2) over substitution.
• Examine option C: ketone + HCN involves CN⁻ nucleophile adding to C=O carbonyl.
• Examine option D: alkane + Cl₂ is free radical substitution.

Why C is correct:

• Propanone (CH₃COCH₃) reacts with HCN via nucleophilic addition, where CN⁻ attacks the electrophilic carbonyl carbon, forming a cyanohydrin (R₂C(OH)CN).

Why the others are wrong:

• A: Electrophilic addition, with H⁺ adding first to the alkene π-bond.
• B: Primarily E2 elimination in ethanolic NaOH, removing HBr to form alkene.
• D: Free radical chain reaction, with Cl• abstracting H from methane.

Final answer: C