Answer
(a) $NH_3$;$N{H_4}^+$ is a Bronsted-Lowry conjugate acid-base pair.
Work Step by Step
A conjugate acid-base pair must differ by only one proton, which means a hydrogen and a difference of 1 charge must be present.
(a) 1 hydrogen and one charge. Correct.
(b) 2 hydrogens and two charges. Incorrect.
(c) 1 halogen. Incorrect.
(d) 1 Oxygen. Incorrect.
