Here's why:

* Conjugate base: A conjugate base is formed when an acid loses a proton (H⁺).

* HPO₄⁻² (dihydrogen phosphate ion): This is an acidic species.

* PO₄⁻³ (phosphate ion): This is formed when HPO₄⁻² loses a proton.

Therefore, PO₄⁻³ is the conjugate base of HPO₄⁻².