Reactivity of carbonyl compounds can be, decided by two factors:

(i) Electronic factor:- greater the number of alkyl groups, lesser will be its electrophilic affect.
(ii) Steric factor:- Lesser the steric factor, greater will be its reactivity.
is most reactive towards nucleophilic addition reactions. Carbonyl compounds are polar with positive charge on carbon atom which is attacked by nucleophiles. 2-ERG in ketones make carbon less deficient than aldehydes. Benzene ring exhibits +R effect which thereby decreases the ease of nucleophilic addition reaction in benzaldehyde & acctophenone. Hence, the reactivity order is-