According to the Bronsted-Lowry Theory:

• A Conjugate Base is the species remaining after an acid has donated a proton ($H^{+}$).
• Given species: $H_{2}P{O}_4^{-}$ Reaction: $H_{2}P{O}_4^{-}\to H^{+}+HP{O}_4^{2-}$
• By removing one proton, the charge decreases by 1 (from $-1$ to $-2$).
• Therefore, $HP{O}_4^{2-}$ is the conjugate base.