The correct answer is chemically bound to hemoglobin.
In the blood, oxygen is combined with hemoglobin, that is, a protein found in the red blood cells. Hemoglobin refers to a protein that can combine with four molecules of oxygen.
Oxygen is not carried in the plasma; it is carried by the red blood cells. These cells comprise of a red element known as hemoglobin that combines with oxygen and transports it around the body in the blood and then lets it go when required.
