The correct option is C p−nitrophenol
Presence of electron withdrawing substituent (like−NO2, −Cl) stabilizes the phenoxide ion more by dispersal of negative charge, thus increases the acidity of phenol. While electron releasing group (like −CH3,−OCH3 etc) destabilizes the phenoxide ion by intensifying the negative charge and thus, decreases the acidity of phenol. Electron withdrawing power of nitro group is more than the chloro group. Hence, p−nitrophenol is most acidic among the given.