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

A conjugate base is formed by the removal of one proton ($H^{+}$) from an acid.
${\text{H}}_2{\text{PO}}_4^{-}\to {\text{H}}^{+}+{\text{HPO}}_4^{2-}$.
Therefore, ${\text{HPO}}_4^{2-}$ is the conjugate base.