The acidity of carboxylic acids depends upon the nature of the alkyl group attached to $ COOH $ . The presence of an electron-withdrawing ( $ -I $ effect) group weakens the $ OH $ bond in the acid and increases the acidity, while the presence of an electron releasing ( $ +I $ effect) group strengthens the $ OH $ bond of acid and thus, decreases the acidity. Thus, the weakest acid is, $ \underset{2-methylpropanoic\text{ }acid}{\mathop{C{{H}_{3}}-\overset{\begin{smallmatrix} C{{H}_{3}} \\ | \end{smallmatrix}}{\mathop{CH}}\,-COOH}}\, $