The correct option is B Capillaries
Capillaries are essential for the delivery of oxygen to the tissues and the exchange of nutrients between blood and interstitial fluid surrounding the cells. This function is well supported by the anatomy of the vessels. The thin walls of the capillaries are composed of a single layer of endothelial cells. As a result, gasses such as oxygen and carbon dioxide can diffuse through their walls, as can lipid soluble substances.
Capillaries also play an important role in regulating the relative volume of the blood and interstitial fluid by allowing a bulk flow through their walls. This exchange of water and solutes occurs in response to the pressure gradient across the capillary wall.
Therefore, the correct answer is option B.