Which of the following has the maximum acidic strength ?

$o$-nitrobenzoic acid is the strongest acid among these. $-\mathrm{NO}_2$ group is a powerful electron withdrawing group and it has most effect when present at $o$-position. It is also known as o-effect. Due to its electron withdrawing nature electron density on H -atom in -COOH group becomes very less, hence it can be easily donated.