The answer is A (\(\text{BH}_3\) and \(\text{BH}_4^-\)).
A Brønsted-Lowry conjugate acid-base pair differs by one proton, \(\text{H}^+\).
• A: \(\text{BH}_3 + \text{H}^- \rightleftharpoons \text{BH}_4^-\). The two species differ by a hydride ion, \(\text{H}^-\), not a proton, so they are not a conjugate pair.
• B is a conjugate pair: \(\text{H}_2\text{PO}_4^- \rightleftharpoons \text{HPO}_4^{2-} + \text{H}^+\), differ by \(\text{H}^+\).
• C is a conjugate pair: \(\text{HCO}_2\text{H}_2^+ \rightleftharpoons \text{HCO}_2\text{H} + \text{H}^+\), differ by \(\text{H}^+\).
• D is a conjugate pair: \(\text{NH}_4^+ \rightleftharpoons \text{NH}_3 + \text{H}^+\), differ by \(\text{H}^+\).
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