Vascular system is made up of blood and lymph carrying tubular networks. These are very essential for carrying oxygen, nutrients, waste material and various types of chemicals.
Arteries (A) are elastic and thick walled. They are capable of vasoconstriction and vasodilation.
Veins (B) are relatively thin walled and relatively inelastic. They carry blood from heart and to heart, respectively.
Arterioles (D) is a small branch of an artery that could lead into capillaries.
All these blood vessels are too thick (their walls are made of too many layers of cells) to enable a direct diffusion of the blood-flow nutrients and chemicals. Therefore, options A, B and D are incorrect.
It is at the stage of the unicellular layered ultra thin-walled capillaries (C) that the oxygen and nutrient exchange with the other cells. They are present as intermediaries between arterioles and venules. Their diameter (of the tube) is like hair-thin.
Therefore, option (C) is correct.