The reactivity of carbonyl compounds towards nucleophilic addition is influenced by the electron-withdrawing or electron-donating groups attached. More reactive compounds are those with fewer electron-withdrawing groups. Thus, the order of reactivity is:
B (HCHO) has the highest reactivity.
C (Cl3CHO) follows due to the electron-withdrawing effect of the -Cl groups.
A (CH3CHO) is less reactive compared to C.
D (CH3COCH2CH3) is the least reactive due to the bulky group.