The correct option is A HPO2−4
According to the Bronsted-Lowry, a conjugate acid-base pair consists of two substances that differ only by the presence of a proton H+. A conjugate acid is formed when a proton is added to a base, and a conjugate base is formed when a proton is removed from an acid.
H2PO−4→HPO2−4(Conjugate base)+H+