The correct option is B Ditert-Butyl ketone
Aldehydes are more reactive than ketones towards nucleophiles due to following reasons:
1. Aldehydes tend to be more reactive to nucleophilic addition than ketones due to less steric hinderance. Ketones have two alkyl groups attached to their carbonyl carbon while aldehydes only have one. This means nucleophiles have a less sterically hindered path when attacking the carbonyl carbon of an aldehyde.
2. Two alkyl groups in ketones reduce the electrophilicity of the carbonyl carbon more effectively than in aldehyde.
Hence, formaldehyde is more reactive toward nucleophilic addition reactions among given options as it sterically less hindered and had more electrophilic carbon center.
Thus, acetaldehyde is more reactive than given ketones.
Now among ketones, the steric effect comes into consideration when comparing these compounds. Ditert-Butyl ketone has two very bulky tert-butyl groups, one on either side of the carbonyl carbon. This increases the hinderance to the incoming nucleophile and decreases the reactivity thus it is least reactive.