Carboxylic acids are more acidic than phenols because

Carboxylic acids dissociate in water to form a carboxylate ion and a proton. The carboxylate ion is highly stabilized by resonance, as the negative charge is delocalized equally over two highly electronegative oxygen atoms, resulting in two equivalent resonance structures.

Phenols dissociate to form a phenoxide ion and a proton. Although the phenoxide ion is also resonance stabilized, the negative charge is delocalized over one oxygen atom and the less electronegative carbon atoms of the benzene ring, resulting in non-equivalent resonance structures.

Since equivalent resonance structures provide greater stability than non-equivalent ones, the carboxylate ion (conjugate base of carboxylic acid) is more resonance stabilized than the phenoxide ion (conjugate base of phenol). Therefore, carboxylic acids are more acidic than phenols.

Answer: More resonance stabilisation of their conjugate base