Final answer:
Ammonia (NH3) acts as a base in aqueous solution by accepting a proton from water to form ammonium (NH4+) and hydroxide ions (OH-), and because it has a lone pair of electrons that can accept a proton. Option A is correct.
Step-by-step explanation:
When ammonia (NH3) is dissolved in a solution, it functions as a Brønsted-Lowry base because it accepts hydrogen ions (protons) from the water. In aqueous solution, ammonia reacts with water to form ammonium ions (NH4+) and hydroxide ions (OH-), as shown in the reaction:
NH3(aq) + H2O(l) ⇒ NH4+(aq) + OH−(aq)
The nitrogen atom in ammonia has a lone pair of electrons that can be used to bond to a hydrogen ion. This attribute of ammonia classifying it as a base is consistent with both the Brønsted-Lowry and Lewis base definitions, the former defining a base as a proton acceptor and the latter defining a base as having an electron pair to donate.
Considering the information above, the correct answer to the student's question is a. a base, because ammonia accepts a hydrogen ion when dissolved in a solution.