CH3−-COOH is strongest acid among all even stronger than
CH3−-COOH due to electrophilic carbonyl group in it.
Among all carboxylic acids given,
CH3−-COOH is strongest acid because it has least
+I effect of alkyl group present.
Alkyl group present in carboxylic acid has electron donating inductive effect, that increases electron density in the
O−H bond. As a result, release of
H+ ions form the carboxylic acid will be more difficult.