$HCN \left(aq\right)\rightleftharpoons H+\left(aq\right) + CN{ }^{-}\left(aq\right)$ . At equilibrium, the addition of CN– ions would

As we know the equilibrium, $HCN \left(aq\right)\rightleftharpoons H^{+}\left(aq\right) + CN{ }^{-}\left(aq\right)$

The concentration of cyanide ions rises when cyanide ions are added to the reaction mixture. More cyanide ions will join with hydrogen ions to cancel out the effect of an increase in the concentration of cyanide ions as the equilibrium shifts backward. As the dissociation of HCN is prevented, this will also lower the hydrogen ion concentration.

Therefore, the correct answer is option (A) decrease H⁺ ion concentration.