Answer
The conjugate base of $HP{O_4}^{2-}$ is $P{O_4}^{3-}$.
Work Step by Step
To find the conjugate base of an acid, just remove one proton $(H^+)$ from the compound.
1. Remove one hydrogen:
$HP{O_4}^{2-} -- \gt PO{_4}^{2-}$
2. Subtract one from the charge
$-2 --\gt -3$
$P{O_4}^{2-} -- \gt P{O_4}^{3-}$