Maximum decarboxylation occurs in: CH3COOH, C6H5COOH, C6H5CH2COOH, CH3COCH2COOH

CH3COCH2COOH is a β-ketoacid. Thus decarboxylation is maximum in a carboxylic acid containing an electron withdrawing group such as >CO or -COOH at the β-carbon atom with respect to the -COOH group. Option D is correct.