Answer choice C is Correct.

Ammonium (NH₄⁺) has a pKa value of 9.26 as given in the chart. This means that when the acid and conjugate base are present in equal molar concentration, the pH of the solution will be 9.26. To make the pH of the solution equal 9.5, the conjugate base must be in excess to such a degree that the log of the base to acid ratio is 0.24. Choice A lists the conjugate base (ammonia) in excess, thus choice A is valid. Hydrocyanic acid (HCN) has a pKa value of 9.32 as given in the chart. This means that when the acid and conjugate base are present in equal molar concentration, the pH of the solution will be 9.32.

To make the pH of the solution equal 9.5, the conjugate base must be in excess to such a degree that the log of the base to acid ratio is 0.18. Choice B lists the conjugate base (sodium cyanide) in excess, thus choice B is valid. From choice A we know that an ammonium/ammonia solution must have excess ammonia for the pH to equal 9.5. If one half equivalent of the strong acid HCl were added to one equivalent of ammonia, then the pH would equal the pKa (9.26). The excess HCl added will lower the pH to a value less than 9.5. This makes choice C invalid and thus the answer you should choose. From choice D we know that an hydrocyanic acid/cyanide solution must have excess cyanide for the pH to equal 9.5. If one half equivalent of the strong base NaOH were added to one equivalent of hydrocyanic acid, then the pH would equal the pKa (9.32). The excess NaOH added will raise the pH to a value greater than 9.32 and approximately equal to 9.5, making choice D valid.