In water, the pKa values are benzoic acid 4.20, 2-chlorobenzoic acid 2.94, 4-chlorobenzoic acid 3.98 and 4-methoxybenzoic acid 4.47. The strongest acid owes its strength mainly to

Athe $+ M$ effect of chlorine in 4-chlorobenzoic acid
Bthe $+ M$ effect of the methoxy group in 4-methoxybenzoic acid
Cthe ortho effect together with the $- I$ effect of chlorine in 2-chlorobenzoic acid
Dhyperconjugation in benzoic acid

Answer: C

The lowest pKa (2.94) belongs to 2-chlorobenzoic acid. The ortho chlorine is close to the carboxyl group, withdraws electrons strongly and forces

COOH

out of the ring plane. The

+ M

donation of methoxy makes 4-methoxybenzoic acid weaker than benzoic acid.