Which of the following is not a conjugate acid- base pair?

A conjugate acid-base pair, according to the Bronsted-Lowry definition of acids and bases, consists of two substances that are unique only in that they contain a proton (H⁺). When a proton is supplied to a base, a conjugate acid is formed, and vice versa when a proton is taken away from an acid, a conjugate base is formed.

Due to its two-proton difference, the acid-base pair$[{\text{H}}_2\text{P}{{\text{O}}_{\text{4}}}^{\text{-}},\text{ P}{{\text{O}}_4}^{3-}]$ is not conjugate.

Only one proton separates a conjugate acid-base pair from another. For example, the only proton that separates HCl and Cl- ion. They are the conjugate acid-base pair.