Correct answer: D. Hemoglobin
The protein hemoglobin has four polypeptide chains, each one bound to a heme group. Each hemoglobin molecule can carry four oxygen molecules because of the presence of four heme groups. The binding of the first oxygen molecule modifies the shape of hemoglobin molecule slightly, this helps in increasing the oxygen binding capacity of the hemoglobin molecule. So, this alteration in shape increases the oxygen carrying capacity of the hemoglobin molecule.
