Explanation

(a)
A carboxylic acid is stronger acid than phenol, hence both III and IV are stronger acids than both I and II. Also IV has a methyl group that gives electrons donating inductive effect and decreases the acid strength. Therefore, III is stronger acid than IV. Between I and II, the dominate electron withdrawing inductive effect of chlorine increases acid strength of phenol slightly, hence II is stronger of phenol slightly, hence, II is stronger acid than I.
Thus, the overall order is: (a) III>IV>II>I.