Consider various species generated when H3PO4 is dissolved in water. According to the Bronsted Lowry’s Concept among these, the conjugate acid of HPO2−4 is :
When a Bronsted-lowry acid is in equilibrium, it will donate a proton (as acids do) in the form of a H+, and therefore become a base (willing to accept a proton). This resulting base is called the conjugate base. Again when in equilibrium, it will accept a proton, and become a conjugate acid.
Here is the equilibrium expression:
HPO2−4+H2O⇌H2PO−4(conjugate acid)+OH−.
Simply the conjugate acid of any species is that species plus a proton; so it's H2PO−4.