Arrange the following acids in order of their increasing pK_a value:…

The correct answer is Option (4) → B < C < A < D

The stronger an acid, the larger its K_a and the smaller its pK_a (pK_a = −log K_a). So increasing pK_a is the order of decreasing acid strength.

A substituent on the ring changes the stability of the benzoate ion (the conjugate base):

• An electron-withdrawing group such as –\( \ce{NO2} \) (−I and −R) spreads the negative charge, stabilises the anion and increases acidity. Its inductive pull is strongest when it is closest to –COOH, so the ortho (2-) isomer is the strongest acid.

• An electron-donating group such as –\( \ce{OCH3} \) at the para position (+R) pushes electron density towards the carboxylate, destabilises it and decreases acidity.

Acid strength: 2-nitrobenzoic > 4-nitrobenzoic > benzoic > 4-methoxybenzoic acid. The pK_a values run the other way:

    2-Nitrobenzoic (2.17) < 4-Nitrobenzoic (3.41) < Benzoic (4.19) < 4-Methoxybenzoic (4.46)

Shortcut: the 4-methoxy acid (D), with an electron-donating group, is the weakest acid and has the highest pK_a, so it comes last. Options (2) and (4) end with D, and only Option (4) starts with the strongest acid B.

NCERT (2026-27 edition):
Effect of Substituents on the Acidity of Carboxylic Acids – “… The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.”

Note: The same/similar question has been asked in different years by NTA as per the details given below: