Explanation

(b)
Phenols are acidic in nature due to resonance stabilisation of phenoxide ion. Presence of electrons releasing groups such as $–CH_{3}$ destabilises ion and decreases the acidic acidic nature of phenols. On the other hand presence of electron withdrawing group in the ring stabilise phenoxide ion and increases the acidic nature of phenols. Further more meta-isomer is less acidic. Then para because it is stabilised by inductive effect only. Thus, correct order is IV > III > I > II