We’re being asked to find the Bronsted-Lowry Base in the given reaction.
Let’s define a Bronsted-Lowry Acid and a Bronsted-Lowry Base. Recall also that Bronsted-Lowry acids and bases occurs in pairs called conjugate acid/base pairs.
Bronsted-Lowry acid → proton (H+) donor
Conjugate base → one less hydrogen form of the Bronsted-Lowry acid
Bronsted-Lowry base → proton (H+) acceptor
Conjugate acid → one hydrogen greater than Bronsted-Lowry base
Given Reaction: H3PO4 + NH3 → NH4+ + H2PO4-