Final answer:
Option (C) (NH₄⁺) and (NH₃) represents a Brønsted-Lowry conjugate acid-base pair.
Step-by-step explanation:
A conjugate acid-base pair is a pair of substances related by the loss or gain of a single hydrogen ion. In this scenario, option (C) (NH₄⁺) and (NH₃) represents a Brønsted-Lowry conjugate acid-base pair. The NH₄⁺ is the conjugate acid of NH₃, and NH₃ is the conjugate base of NH₄⁺. Every Brønsted-Lowry acid-base reaction can be labeled with two conjugate acid-base pairs.