The increasing order of the rate of HCN addition to compound A — D is 
(A) HCHO  (B) CH₃COCH₃  (C) PhCOCH₃  (D) PhCOPh 

Addition of HCN to carbonyl compounds is nucleophilic addition reaction. The order of reactivity of carbonyl compounds is Aldehydes (smaller to higher) Ketones (smaller to higher), Then 
$\mathrm{PhCOPh}<\mathrm{Ph}.{\mathrm{COCH}}_3<{\mathrm{CH}}_3{\mathrm{COCH}}_3<\mathrm{HCHO}$
The lower reactivity of ketones is due to presence of two alkyl group which shows +I effect. The reactivity of ketones decreases as the size of alkyl group increases.