Consider the acidity of the following carboxylic acids:
(1) $\text{PhCOOH}$
(2) $\text{o}–{\text{NO}}_2{\text{C}}_6{\text{H}}_4\text{COOH}$
(3) $\text{p}–{\text{NO}}_2{\text{C}}_6{\text{H}}_4\text{COOH}$
(4) $\text{m}–{\text{NO}}_2{\text{C}}_6{\text{H}}_4\text{COOH}$
Which one is the most acidic?

For substituted benzoic acids, electron-withdrawing groups (like $-{\text{NO}}_2$) increase acidity.
The ortho-effect significantly increases the acidity of ortho-substituted benzoic acids regardless of the nature of the group (electron-withdrawing or donating) compared to meta and para isomers. Thus, $o$ -nitrobenzoic acid is the strongest acid among the given options.