Assuming equal concentrations of conjugate base and acid, which one of the following mixtures is suitable for making a buffer solution with an optimum pH of 9.2 - 9.3?
a. CH3COONa / CH3COOH (Ka = 1.8 ´10-5)
b. NH3 / NH4Cl (Ka = 5.6 ´10-10)
c. NaOCl / HOCl (Ka = 3.2 ´10-8)
d. NaNO2 / HNO2 (Ka = 4.5 ´10-4)
e. NaCl / HCl