More the stability of conjugate bases, more is the acidity of the acid. $\mathrm{Ph}, \mathrm{Br}, \mathrm{NO}_2$ exerts the $-\mathrm{I}$ effect to the $\mathrm{C}_{\propto}$ which, further, stabilizes the carboxylate ion.
Order of $(-\mathrm{I})$ effect of the substituents :
$\mathrm{H} < \mathrm{Ph} < \mathrm{Br} < \mathrm{NO}_2$