Which of the following compounds is the strongest acid?

The correct answer is Option (3) → 4-Nitrobenzoic acid

A group on the ring that withdraws electrons stabilises the benzoate ion and makes the acid stronger; a group that donates electrons makes it weaker. The smaller the pK_a, the stronger the acid.

The \( \ce{-NO2} \) group is strongly electron withdrawing, so 4-nitrobenzoic acid (pK_a 3.41) is the strongest acid of the four.

Why the other options are incorrect

Option (1) Benzoic acid — Incorrect. With no substituent on the ring, its pK_a is 4.19, higher than 3.41, so it is weaker than 4-nitrobenzoic acid.

Option (2) 4-Methylbenzoic acid — Incorrect. The \( \ce{-CH3} \) group is electron donating, so this acid is weaker even than benzoic acid.

Option (4) 4-Methoxybenzoic acid — Incorrect. The \( \ce{-OCH3} \) group is electron donating; its pK_a is 4.46, higher even than that of benzoic acid.

NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “… The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.”