The correct option is C HPO2−4
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-
H2SO4→HSO−4+H+
Therefore,
Conjugate base of H2SO4 is HSO−4.
Using this logic, for the given example:
H2PO−4→HPO−24+H+
Hence,
Conjugate base of H2PO−4 is HPO−24.