A Buffer May Be Prepared by Mixing a Weak Acid

A buffer may be prepared by mixing a weak acid with a roughly equivalent amount of strong base.Which of the acids below is best for the preparation of a buffer with a pH of 4.00?

A) sulfurous acid ,H₂SO₃;K_a = 1.7 × 10−²
B) hydrofluoric acid,HF;K_a = 6.9 × 10−⁴
C) benzoic acid,HC₇H₅O₂;K_a = 6.6 × 10−⁵
D) dihydrogen phosphate ion,H₂PO₄−;K_a = 6.2 × 10−⁸
E) ammonium ion,NH₄⁺;K_a = 5.6 × 10−¹⁰