Cyanohydrin formation mechanism Steps:

• KCN dissociates to provide CN⁻ nucleophile in the HCN buffer system.
• CN⁻ attacks the electrophilic carbonyl carbon of the aldehyde (ethanal, likely intended over ethanol).
• This forms a tetrahedral alkoxide intermediate, R-CH(O⁻)CN.
• Protonation of the alkoxide by HCN yields the hydroxynitrile product, regenerating CN⁻. Why B is correct:
• B describes nucleophilic addition of CN⁻ to the carbonyl, the rate-determining step per organic mechanism principles. Why the others are wrong:
• A: Protonation of carbonyl occurs in acid-catalyzed paths, not base-assisted here.
• C: Deprotonation of HCN is prior equilibrium, not the first mechanistic step.
• D: CN⁻ departure is irrelevant, as this is addition, not substitution.

Final answer: B