Correct Option: B

Hemoglobin is the iron-containing oxygen-transport metalloprotein in the red blood cells of all vertebrates which carries oxygen from the respiratory organs to the rest of the body. It changes shape when it binds oxygen. When it changes shape, it absorbs different wavelengths of light, making it change color. When blood is exposed to air, much more of the hemoglobin absorbs oxygen than had in the vein the blood came from. Therefore, the blood turns red.