The conjugate base of ${\mathrm{H}}_2{\mathrm{PO}}_4^{-}$ is

species formed by removing ${\mathrm{H}}^{+}$ from given species is conjugate base.

${\mathrm{H}}_2{\mathrm{PO}}_4^{-} \to {\mathrm{H}}^{+} + {\mathrm{HPO}}_4^{2-}$

  Acid                       Base