Which is the strongest acid among the following compounds?

The correct answer is Option (2) → C₆H₅COOH

An acid is stronger when its carboxylate ion is more stable. In benzoic acid the –COOH group is attached directly to an sp² carbon of the ring; this more electronegative carbon draws electron density away and stabilises the benzoate ion. Benzoic acid also has no electron-donating group on the ring, so it is the strongest of the four.

Why the other options are incorrect

Option (1) C₆H₅CH₂COOH — Incorrect. A CH₂ group separates the ring from the carboxyl group, so the –COOH is on an sp³ carbon and the ring’s effect is weaker; phenylacetic acid is weaker than benzoic acid.

Option (3) m-CH₃C₆H₄COOH — Incorrect. –CH₃ is an electron-donating group; it destabilises the carboxylate ion and lowers the acidity.

Option (4) p-CH₃C₆H₄COOH — Incorrect. The electron-donating –CH₃ group again makes this acid weaker than benzoic acid.

NCERT (2026-27 edition):
8.9.1 Reactions Involving Cleavage of O–H Bond – “Direct attachment of groups such as phenyl or vinyl to the carboxylic acid, increases the acidity of corresponding carboxylic acid … This is because of greater electronegativity of sp² hybridised carbon to which carboxyl carbon is attached. The presence of electron withdrawing group on the phenyl of aromatic carboxylic acid increases their acidity while electron donating groups decrease their acidity.”
“… C₆H₅COOH > C₆H₅CH₂COOH > CH₃COOH > CH₃CH₂COOH”