The conjugate base of H2​PO4−​ is:

A

HPO42−​

B

P2​O5​

C

H3​PO4​

D

PO43−​

The concept of conjugate Acid-Base pair was given by Lowry-Bronsted Theory.
According to the theory:
Acids are proton(H+)donors.
Bases are proton(H+)acceptors.
Hence, in order to get the conjugate base of a species, we should consider that substance as an acid and vice versa.
For example,
H2​SO4​→HSO4−​+H+
Therefore,conjugate base ofH2​SO4​isHSO4−​.
Using this logic, for the given example:
H2​PO4−​→HPO4−2​+H+
Hence, conjugate base ofH2​PO4−​isHPO4−2​.