Which of the Following Weak Acids Would Make the Best

Which of the following weak acids would make the best buffer at pH = 5.0?

A) acetic acid (K_a = 1.74 × 10−⁵, pK_a = 4.76)
B) H₂PO₄− (K_a = 1.38 × 10−⁷, pK_a = 7.20)
C) bicarbonate (K_a = 6.3 × 10−¹¹, pK_a = 10.24)
D) tris-hydroxymethyl aminomethane (K_a = 8.32 × 10−⁹, pK_a = 8.07)
E) lactic acid (K_a = 1.38 × 10−⁴, pK_a = 3.86)