The correct option is A 9.3
Milli-moles of salt KCN=2.5×500=1250
Milli-moles of HCN=2.5×500=1250
Therefore in the buffer solution [Anion of Salt] = [Acid]
So, this quantity is the ratio of concentration of the anion of the salt and the acid present in the mixture will be equal to 1.
[Anion of Salt][Acid]=1
pH=pKa+log10([Anion of Salt][Acid])
or, pH=pKa
pKa+pKb=14 at 25oC
pKa of HCN=14−pKb=14−4.7=9.3pH=pKa=9.3