Functions in diffusion
a. artery
b. capillary
c. vein

b. capillary

Explanation:

Blood capillaries are the smallest vessels of the circulatory system (5 to 10 μm). It is in them that the gas exchange between the blood and body cells occurs (in the process called diffusion), as well as the delivery of nutrients and the excretion uptake of metabolism. The capillaries are extremely thin, formed only by a layer of endothelial tissue.

The capillaries located in the lungs are responsible for the diffusion process where it allows the red blood cells to release carbon dioxide collected by the systemic circulation and carry oxygen molecules back into the body. Red blood cells have a larger diameter than the blood capillaries themselves, so during their passage they undergo a slight change in shape to be able to flow through the capillaries.

option number one would be youre best option