In the reaction $\mathrm{HCN}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{CN}}^{-}$ the conjugate acid-base pair is

In the reaction $\mathrm{HCN}+{\mathrm{H}}_2\mathrm{O}\rightleftharpoons {\mathrm{H}}_3{\mathrm{O}}^{+}+{\mathrm{CN}}^{-}$ the conjugate acid-base pair is $\mathrm{HCN}, {\mathrm{H}}_3{\mathrm{O}}^{+}$

A conjugate acid-base pair is a pair of substances that differ exclusively in the presence of a proton (${\mathrm{H}}^{+}$) according to the Bronsted-Lowry definition of acids and bases. A proton given to a base creates a conjugate acid, while a proton taken away from an acid creates a conjugate base.

Hence the correct option is D.