Explanation

(a)
Carbonyl compounds undergoes nucleophilic addition reaction.
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If group or atom attached with carbonyl carbon shows negative inductive effect, then it decreases electron density on carbonyl carbon and facilitate the attack of nucleophile, hence reactivity of carbonyl compound increases. The aromatic aldehydes and ketones are less reactive than their aliphatic analogues due to +R effect of benzene ring. The increasing order of the nucleophilic addition reaction in the following compounds will be.
$CH_{3}CHO > CH_{3}COCH_{3} > PhCOCH_{3} > PhCOPh$