Explanation

(b)
Proton donors are acids. The acidity of phenol increases by presence of electron withdrawing groups (e.g.,$- NO_{2}$ group) because these groups weaken the $O - H$ bond and stabilise the phenoxide by resonance. More the number of electron withdrawing group in compound more will be acidity. On the other hand electron donating group (e.g., $CH_{3}$) decrease the acidity of phenol because they strengthen the $O - H$ bond.
Therefore, correct order of acidity is
III > IV > I > II
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