Q.$H_2PO_4^-$ is the conjugate base of

Proposed independently by Lowry and Bronsted in 1923, this theory defines acids and bases by proton transfer rather than by what they release in water: an acid is a substance with a tendency to donate a proton to another substance, and a base is a substance with a tendency to accept a proton from another substance -- an acid is a proton donor, a base is a proton acceptor.

When $HCl$ dissolves in water, $HCl+H_2O \rightarrow H_3O^++Cl^-$: $HCl$ donates a proton (acts as the acid) and $H_2O$ accepts it (acts as the base). When $NH_3$ dissolves in water, $H_2O+NH_3 \rightleftharpoons NH_4^++OH^-$: $NH_3$ accepts a proton (acts as the base) and $H_2O$ donates it (acts as the acid) -- so water can play either role depending on its partner. …