All the six carbonyl compounds are less reactive than formaldehyde with respect to nucleophilic addition. The reactivity of the carbonyl group towards the nucleophilic addition reactions depends upon the magnitude of the positive charge on the carbonyl carbon atom (electrophilic character of carbonyl carbon) and also, on crowding around the carbonyl carbon atom (steric effect). Both these factors are in favour of formaldehyde. Hence, the reactivity of formaldehyde towards nucleophilic addition is maximum. The correct order of reactivity towards nucleophilic addition is as follows:
Formaldehyde > Aldehyde > Ketone