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

A

HPO32−​,PO33−​

B

H2​PO4−​,H​PO42−​

C

H2​PO4−​,H3​PO4​

D

H2​PO4−​,PO33−​

H2​PO4−​,(PO3)3−​is not a conjugate acid-base pair as it differs by two protons.
A conjugate acid-base pair differs by one proton only. For example,HClandCl−ion differ by one proton only. Hence, they are the conjugate acid-base pair.
Thus,(A)HPO32−​,(PO3)3−​, (B)H2​PO4−​,H​PO42−​and (C)H2​PO4−​,H3​PO4​are conjugate acid base pairs as they differ by one proton only.