Consider the reversible reaction $HC{N}_{aq}\rightleftharpoons H_{aq}^{+}+C{N}_{aq}^{-}$ }. At equilibrium, the addition of $C{N}_{aq}^{-}$would

The equilibrium reaction is

${\mathrm{HCN}}_{\mathrm{aq}}\rightleftharpoons {\mathrm{H}}_{\mathrm{aq}}^{+}+{\mathrm{CN}}_{\mathrm{aq}}^{-}$

When cyanide is added to the reaction mixture, the concentration of cyanide ions increases. The equilibrium will shift in the backward direction so that more cyanide ions will combine with hydrogen ions to nullify the effect of increased cyanide ion concentration. This will also decrease the hydrogen ion concentration, as dissociation of HCN is suppressed.

So, the correct option is (B).