A carbonyl compound reacts with HCN to form a cyanohydrin which on hydrolysis forms a racemic mixture of hydroxy acids. The compound is 

Reaction of $HCN$ with a carbonyl compound is called as nucleophilic addition reaction. Acetaldehyde reacts with $HCN$ to form cyanohydrin which on hydrolysis give racemic mixture of alpha-hydroxy acid. As the cyanohydrin carbon has chiral carbon present. 

Hence, correct answer is option (B) Acetaldehyde.