Final answer:
In the reaction H₂O + NH₃ → OH- + NH₄+, NH₃ (ammonia) is acting as a base by accepting a proton from water, making water the acid.
Step-by-step explanation:
According to the reaction H₂O + NH₃ → OH- + NH₄+, the molecule acting as a base is NH₃ (ammonia). A base is defined in the Brønsted-Lowry theory as a substance that can accept a proton (H+), which is what ammonia does here. It accepts a proton from a water molecule, resulting in the formation of NH₄+ (ammonium).
Correspondingly, water acts as a Brønsted-Lowry acid because it donates a proton to the NH₃. This is highlighted by the production of OH- as the conjugate base of water, and NH₄+ as the conjugate acid of ammonia.