The conjugate base of H2​PO4−​ is:

A

HPO4^2−​

B

P2​O5​

C

H3​PO4​

D

PO4^3−​

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 of H2​SO4​ is HSO4−​.

Using this logic, for the given example:
H2​PO4−​→HPO4^−2​+H+

Hence, conjugate base of H2​PO4−​ is HPO4^−2​.

Option A is correct.