Which of the following compounds is most reactive towards nucleophilic addition reactions? 

Carbonyl compounds undergoes nucleophilic addition reaction. 

If froup or atom attached with carbonyl carbon shows negative inductive effect, then it decreases electron density or carbonyl carbon and facilitate the attack of nucleophiles, hence reactive of carbonyl compound increases. (image ii)

If group or atom attached with carbonyl carbon shows positive inductive effect, then it increases electron density or carbonyl carbon and supress the attack of nucleophiles, hence reactivity of carbonyl compound decreases.

The aromatic aldehydes and ketones are less reactive than their aliphatic analogues. This is due to the +R effect of benzene ring.

On the basis of above informations, the increasing order of the nucleopihilic addition reaction in the following compound will be,

CH₃​CHO>CH₃​COCH₃​>PhCOCH_3​>PhCOPh