which one of the following is a buffer solution?

a) 0.40 M HCN and 0.10 KCN

b) 0.20 CH3COOH

c) 1.0 M HNO3 and 1.0 NaNO3

d) 0.10 M KCNe) 0.50 M HCl and 0.10 NaCl