The correct answer is A. Bound to hemoglobin.
Hemoglobin is a protein found in red blood cells that carries oxygen from the lungs to the tissues. It is made up of four subunits, each of which contains a heme group. The heme group is a complex of iron and protoporphyrin IX, which is a ring of four pyrrole rings. The iron atom in the heme group can bind to oxygen, and each hemoglobin molecule can carry up to four oxygen molecules.
When hemoglobin binds to oxygen, the iron atom changes from a ferrous (Fe2+) state to a ferric (Fe3+) state. This change in oxidation state causes the heme group to twist, which exposes the oxygen-binding site to the bloodstream. The oxygen then binds to the heme group and is transported to the tissues.
In the tissues, the oxygen is released from hemoglobin and diffuses into the cells. The oxygen is then used in cellular respiration, which produces energy. The carbon dioxide produced by cellular respiration is then transported back to the lungs, where it is exhaled.
B. Dissolved in plasma is incorrect because only a small amount of oxygen is transported in this way. C. Dissolved in water is incorrect because oxygen is not very soluble in water. D. In ionic form as solute in plasma is incorrect because oxygen is not an ion.