In Brønsted-Lowry theory, a conjugate acid-base pair consists of two species related by the gain or loss of exactly one proton (H^+). Identifying the pairs in H_2O + NH_3 \rightleftharpoons OH^- + NH_4^+: H_2O loses a proton to become OH^-, so H_2O (acid) and OH^- (conjugate base) form one conjugate pair. NH_3 gains a proton to become NH_4^+, so NH_3 (base) and NH_4^+ (conjugate acid) form the second conjugate pair. The correct notation lists acid first then conjugate base: H_2O/OH^- and NH_3/NH_4^+. Option A (H_2O/NH_4^+ and NH_3/OH^-) mixes species across reaction sides incorrectly; these pairs are not related by a single proton transfer. Option C reverses the notation, listing NH_3 paired with OH^- which are not in a conjugate relationship (they don't differ by one proton). Option D reverses the acid-base labels within each pair, listing base before acid. The Brønsted-Lowry conjugate pair concept is fundamental in NCERT Equilibrium and regularly tested in JEE Main. Plausibility check: checking that each pair differs by exactly one H^+: H_2O - H^+ = OH^- ✓, NH_3 + H^+ = NH_4^+ ✓, confirming the conjugate pair assignments.