Explanation

(b)
Due to resonance; the carbonyl group of benzoic acid is coplanar with the ring. If the electron withdrawing substituent (i.e., $- I\$showing) is present at $ortho$ position, it prevents the coplanarity and thus, the resonance. Hence, makes the acid more stronger.
Thus, among the given acids, $ortho$ hydroxy benzene acid is the most acidic.