21) D

We know that at the alveolus, oxygen diffuses out of the alveolus and carbon dioxide diffuses into the  alveolus. 

Since oxygen diffuses out of the alveolus and into the capillary, its concentration must be high in the alveolus since diffusion is movement of particles from high concentration to low concentration. 

So first column must be high. 

Since oxygen diffuses into, its concentration must be low in the capillary since diffusion is movement of particles from high concentration to low concentration. 

So second column must be low.

Since carbon dioxide diffuses into the alveolus, its concentration must be low in the alveolus since diffusion is movement of particles from high concentration to low concentration. 

So third column must be low. 

Since carbon dioxide diffuses out of the capillary, its concentration must be high in the capillary since diffusion is movement of particles from high concentration to low concentration. 

So fourth column must be high. 

So D is the answer.