Final answer:
Carbon dioxide is primarily transported in the blood as bicarbonate ions in plasma, alongside two other mechanisms including dissolution in blood and binding to hemoglobin.
Step-by-step explanation:
The primary form in which carbon dioxide is carried in blood is b. As a bicarbonate ion in plasma. In the blood, carbon dioxide can be transported by dissolution in the blood, binding to hemoglobin, or through conversion into bicarbonate ions. The majority of carbon dioxide is transported as part of the bicarbonate system where it diffuses into red blood cells and is quickly converted to carbonic acid (H2CO3) by the enzyme carbonic anhydrase. This carbonic acid then dissociates into bicarbonate ions (HCO3-) and hydrogen ions (H+), with the bicarbonate ions being moved into the plasma.