IIT JEE Advanced 2009 Paper 1

We know that, carboxylic acids are more acidic than phenols.
Further, presence of electron withdrawing groups on the ring increases the acidic nature and electron releasing group decreases the acidic strength.
So, structure III, due to the presence of carboxylic acid is the strongest acid (pKa = 4.17). In structure IV, the +I effect of alkyl group reduces the acidic strength (pKa = 4.37) of the carboxylic acid. Structure III, due to the presence of electron withdrawing substituent (Cl), is more acidic than phenol (pKa = 9.38). Phenol (structure I) is the weakest acid amongst all (pK= 9.98).