Correct Option: D

The most important buffer for maintaining acidbase balance in the blood is the carbonic-acid-bicarbonate buffer. Three different buffer systems exist in blood, the bicarbonate buffer and the phosphate buffer are composed of "simple" chemicals. In addition the carbonyl groups (-COOH) and the amide group (NH₂) present on proteins allow some of these to act as buffers.
H₂CO₃ + H₂O ↔ HCO₃ ^- +H₃O⁺

H₂PO₄^- +H₂O ↔ HPO₄H₃O 2⁺