Answer
The conjugate acid of $H_2P{O_4}^-$ is:
$H_3PO_4$
Work Step by Step
To find the conjugate acid of a base, you just need to add a proton $(H^+)$ to the compound.
1. Add one hydrogen.
$H_2PO{_4}^- -- \gt H_3PO{_4}^-$
2. Add "+1" to the charge of the compound:
$-1 -- \gt 0$
$H_3{PO_4}^- --\gt H_3PO_4$
