Q.What is conjugate Acid-Base pairs ?

$HPO_4^{2-}$ is amphiprotic: it retains an ionisable H to donate ($\rightarrow PO_4^{3-}$, acid behaviour) and a lone pair able to accept a proton ($\rightarrow H_2PO_4^-$, base behaviour), unlike the other species listed.

$HCl$ is a strong Bronsted acid that only donates $H^+$ (to give $Cl^-$); it has no basic character under normal conditions. $Br^-$ is the conjugate base of $HBr$ — it can only accept a proton (to reform $HBr$), having no H to donate. $SO_4^{2-}$ can only accept a proton (to form $HSO_4^-$); it has no acidic H of its own, so it can only act as a base. $HPO_4^{2-}$ sits in the middle of the triprotic phosphoric-acid ladder ($H_3PO_4 \rightleftharpoons H_2PO_4^- \rightleftharpoons HPO_4^{2-} \rightleftharpoons PO_4^{3-}$): …