Phenol is most acidic of all the given compounds. In phenol, the electron withdrawing phenyl ring polarizes the O— H bond thereby facilitating the release of H as H+ and hence phenol is most acidic. In the
electron withdrawing effect of phenylring is somewhat diminished by the −CH2 group and it is therefore less acidic than phenol. In (c) and (d), — OH group is attached to alkyl groups which, due to their +I effect reduce the polarity of — OH bond and so the acidic strength is low.