Buffer systems resist changes in pH when acids or bases are added. In biological systems, effective buffer systems are crucial for maintaining homeostasis. The carbonic acid-bicarbonate system is the most important buffer in blood because CO₂ (which forms H₂CO₃) is constantly produced by metabolism and can be regulated by respiration. Option A represents an acetate buffer, Option B represents a phosphate buffer system (important in cells but less so as the primary blood buffer), and Option D represents a phosphate buffer system.