Which of the following mixtures would made the most effective buffer?

a.....100. mL 5.0M HF/0.05M NaF
b....100. mL 0.05M HF/0.05M NaF
c....500. mL 5.0M HF/ 5.0M Nal
d....100. mL 5.0M HF/5.0M NaF

I don't see the difference between a and b. Generally, the more concentrated the base and acid AND the more they are close to the same total moles, the more effective they are over a larger range. Based on that I would pick d.