Which of the Following Combinations Would Be Best to Buffer $\times$

Multiple Choice

Which of the following combinations would be best to buffer an aqueous solution at a pH of 9.04?

A) H₃PO₄ and H₂PO₄^-,K_a1 = 7.5 $\times$ 10^-3
B) HNO₂ and NO₂^-,K_a = 4.5 $\times$ 10^-4
C) CH₃CO₂H and CH₃COO^-,K_a = 1.8 $\times$ 10^-5
D) H₂PO₄^- and HPO₄^2-,K_a2 = 6.2 $\times$ 10^-8
E) NH₄⁺ and NH₃,K_a = 5.7 $\times$ 10^-10