In the given reaction, HCl donates a proton (H+) to H2O to form H3O+ and Cl-. The species that accepts the proton is the base, which in this case is water (H2O). The species that donates the proton is the acid, which in this case is HCl. The conjugate base is the remaining species after the acid has donated the proton, which is Cl- in this reaction. Therefore, the correct answer is "conjugate base".