Explanation

(a)
Benzoic acid, o-phthalic acid and acetophenone inspite of having double bonds, does not give unsaturation test (addition with $Br_{2}/CCl_{4}$) as they are aromatic compounds and are quite stable due to large resonance energy. Cinnamic acid, on the other hand has a double bond outside the benzene ring (in the side chain) hence it gives unsaturation test.
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