Final answer:
The correct answer is option D. a gas in the blood.
Step-by-step explanation:
The majority of carbon dioxide in the blood is transported as bicarbonate, which is formed in erythrocytes. Carbon dioxide combines with water, with the help of the enzyme carbonic anhydrase, to form carbonic acid. This carbonic acid then dissociates into bicarbonate and hydrogen ions.
Bicarbonate is transported across the cell membrane and into the plasma, while at the pulmonary capillaries, bicarbonate re-enters the erythrocytes and is converted back into carbon dioxide and water.