Carbon dioxide is carried in the blood, mainly as:

Explanation

Nearly 20-25 percent of CO2 is transported by RBCs whereas 70 percent of it is carried as bicarbonate. About 7 percent of CO2 is carried in a dissolved state through plasma. RBCs contain a very high concentration of the enzyme, carbonic anhydrase, and minute quantities of the same is present in the plasma too. This enzyme facilitates the following reaction in both directions. At the tissue site where the partial pressure of CO2 is high due to catabolism, CO2 diffuses into blood (RBCs and plasma) and forms HCO3 and H+. This HCO3 combines with Na+ to form sodium bicarbonate. At the alveolar site where pCO2 is low, the reaction proceeds in the opposite direction leading to the formation of CO2 and H2 O. Thus, CO2 trapped as bicarbonate at the tissue level and transported to the alveoli is released out as CO2. So, the correct option is 'Sodium bicarbonate'.