Cyanohydrin formation via nucleophilic addition Steps:

• Recognize ethanol likely refers to ethanal (acetaldehyde, CH₃CHO) in this context, as alcohols do not form cyanohydrins.
• The reaction is CH₃CHO + HCN → CH₃CH(OH)CN, catalyzed by KCN (provides CN⁻).
• CN⁻ acts as nucleophile, attacking the electrophilic carbonyl carbon.
• This addition forms a new bond between the carbonyl carbon and the carbon of CN, with subsequent protonation of oxygen.

Why B is correct:

• Cyanohydrin formation follows nucleophilic addition mechanism where CN⁻ bonds to the carbonyl C, creating a C-C σ-bond (per organic reaction principles).

Why the others are wrong:

• A: No C-H bond breaks; the carbonyl C=O undergoes addition without H loss from carbon.
• C: No C-N bond exists or breaks; the CN group adds intact as a unit.

Final answer: B