The acid and base are the proton donor and acceptor pairs. The base-conjugate acid pair in the equation has been [tex]\rm NH_3/NH_4[/tex].
The conjugate acid has been the proton donor in the reaction, while the conjugate base has been the proton acceptor in the reaction.
The given equation has been:
[tex]\rm NH_3\;+\;HNO_3\;\rightarrow\;NH_4\;+\;NO_3^-[/tex]
The [tex]\rm NH_3[/tex] group in the equation accepts the protons and has been converted to the [tex]\rm NH_4[/tex]. Thus, it has been the base.
The conjugate acid to the [tex]\rm NH_3[/tex] group in the equation that has been the proton donor has been [tex]\rm NH_4[/tex].
Thus, the conjugate acid and base pair in the equation have been [tex]\rm NH_3/NH_4[/tex]. Thus, option A is correct.
Learn more about the conjugate base, here:
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