The correct option is D Capillaries
Capillaries are the smallest of the body’s blood vessels. They are made of a single layer of endothelial cells and are the sites of transfer of oxygen and other nutrients from the bloodstream to other tissues in the body; they also collect carbon dioxide and waste materials for return to the veins.
Both the arteries and veins are made up of three layers - Tunica interna, Tunica media and the Tunica externa.