Answer:
i think the answer should be b.
the alveolus is the site of exchange of gases (carbon dioxide and oxygen) between the body cells and the alveolar air. diffusion requires a concentration gradient, which basically means that there has to be a difference in concentration of the substance (in this case it's oxygen) between the 2 areas (in this case it's the body cells and the alveolar air) hence, there is more oxygen in the alveolar air than the body cells (this difference in concentration is what allows diffusion of oxygen from a region of HIGHER to LOWER concentration!) this allows oxygen to diffuse from alveolus to body cells, giving your body the oxygen it needs!
btw keep in mind that i am by no means an expert in biology (because im still a student), but b is the most logical answer to me. good luck!