Explanation:
We know that conjugate base of an acid, any acid is defined as the acid less of proton.
The conjugate acid of a base, any base is defined as the base plus a proton.
Now,
Acid + Base $=$ Conjugate acid + Conjugate base
Acid donates hydrogen ions and the base is an acceptor of protons.
The reaction of hydrogen phosphate ion with a proton to form its conugate acid is given below-
$\mathrm{HPO}_4^{2-}+\mathrm{H}^{+} \rightarrow \mathrm{H}_2 \mathrm{PO}_4^{-}$
Hence, the conjugate acid that is formed is $\mathrm{H}_2 \mathrm{PO}_4^{-}$, which is dihydrogen phosphate ion.