Hint: Resonating structure of carboxylate ion is more stable than the resonating structure of phenoxide ion

Resonance structures of phenoxide ion are:

It can be observed from the resonance structures of phenoxide ions that in II, III, and IV, less electronegative carbon atoms carry a negative charge. Therefore, these three structures contribute negligibly toward the resonance stability of the phenoxide ion.

Hence, these structures can be eliminated. Only structures I and V carry a negative charge on the more electronegative oxygen atom.

Resonance structures of carboxylate ions are:

In the case of carboxylate ions, resonating structures I′ and II′ contain a charge carried by a more electronegative oxygen atom.

Further, in resonating structures I′ and II′, the negative charge is delocalized over two oxygen atoms.

But in resonating structures I and V of the phenoxide ion, the negative charge is localized on the same oxygen atom.

Therefore, the resonating structures of carboxylate ion contribute more towards its stability than those of phenoxide ion. As a result, carboxylate ion is more resonance-stabilized than phenoxide ion.

Hence, the carboxylic acid is a stronger acid than phenol.