A Bronsted-Lowry base is a substance that can accept a proton (H+). NH3 can accept a proton to form NH4+ (ammonium ion), so it is a Bronsted-Lowry base.

The reaction of NH3 with H+ to form NH4+ can be represented as follows:

NH3 + H+ → NH4+

In this reaction, NH3 accepts a proton from H+ to form NH4+. Therefore, NH3 is a Bronsted-Lowry base.