Question

Identify the conjugate acid/base pairs in, NH₃ + H₂PO₄^- →  NH₄⁺ + HPO₄^2-

Answer 1

d. acid $H_3PO_4$ conjugate base $H_2PO_4^−$ ; base $NH_3$ conjugate acid $NH_4^+$

The rule is : acid donates the proton, forming the conjugate base, and the base accepts the proton, forming the conjugate acid.