Electron withdrawal group is acid strengthening group. Due to ortho effect, $o-{\mathrm{NO}}_2{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{COOH}$ is the strongest acid.

$\mathrm{p}-{\mathrm{NO}}_2{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{COOH}$ is more acidic than$m-{\mathrm{NO}}_2{\mathrm{C}}_6{\mathrm{H}}_4\mathrm{COOH}$, because the former generates positive charge on carbon bearing —$\mathrm{COOH}$ group whereas the latter generates positive charge on the next carbon atoms.

Thus, the correct order is