Most reactive towards nucleophilic addition is 

Nucleophilic addition reaction is addition of negatively charged species on less negatively charged centre or (electrophilic position). 

Acetophenone is a ketone hence it is less reactive. 

p-tolualdehyde has electron donating methyl group at para position in benzene ring which deactivates the ring towards nucleophilic addition whereas 4-nitrobenzaldehdye has electron withdrawing group nitro present at para position which activates benzene ring towards nucleophilic addition reaction. 

Hence, the correct answer is option (B) 4-nitrobenzaldehyde .