Step-by-step explanation:
H₂PO₄⁻ (aq) + NH₃ (aq) ---> HPO₄²⁻ (aq) + NH₄⁺ (aq)
acid base conjugate base conjugate acid
H₂PO₄⁻ is donating a proton to NH₃. So H₂PO₄⁻ is acting like an acid, and NH₃ is acting as the base. Then, HPO₄²⁻ is the conjugate base of H₂PO₄⁻ and NH₄⁺ is the conjugate acid of NH₃.
Answer: b) acid