The increasing order of the rate of $\text{HCN}$ addition to compounds A – D is:(A) $\text{HCHO}$ (B) ${\text{CH}}_3{\text{COCH}}_3$ (C) ${\text{PhCOCH}}_3$ (D) $\text{PhCOPh}$

Nucleophilic addition to carbonyl groups is influenced by steric and electronic factors.Electronic factor: Electron-donating groups (like alkyl or phenyl) decrease the positive charge density on the carbonyl carbon, making it less reactive. Phenyl groups provide more deactivation than methyl groups due to resonance.Steric factor: Bulkier groups around the carbonyl carbon hinder the approach of the nucleophile (${\text{CN}}^{-}$).$\text{HCHO}$ (A) is the most reactive (no alkyl groups).${\text{CH}}_3{\text{COCH}}_3$ (B) is next.${\text{PhCOCH}}_3$ (C) is less reactive due to the phenyl group.$\text{PhCOPh}$ (D) is the least reactive due to two bulky phenyl groups and resonance stabilization.Order: D < C < B < A.