When we have an acid-base reaction, the acid will be that substance that donates protons, in the form of H+ ions. The base will be the one that accepts the H+ ions. Now, the base that accepts the protons becomes a potential proton donor, thus becoming a conjugate acid. This is the case with H3O+.
H2O accepts H+ ions, so it is a base and when it becomes H3O+ it becomes a potential H+ proton donor, that is, a conjugate acid.
so, the answer will be B. Conjugate acid
