Why diffusion is insufficient to meet the oxygen requirements of multicellular organisms like humans? 

Concept: Diffusion is the process through which particles move from a gradient of higher concentration to a gradient of lower concentration. It is a physical process. 

Although this movement of molecules across the concentration gradient can be facilitated by some other molecules, it does not necessarily require any external energy molecules to take place. The rate at which molecules are diffusing depends on the nature of the material and the medium that are involved in the diffusion process. Diffusion also depends on certain factors including the temperature, area of interaction, size of the particle involved and the depth of the concentration gradient. Humans are complex organisms and they are made up of several voluminous cells that are not directly in contact with the external environment hence diffusion is not sufficient to meet the body’s oxygen requirement. 

Hence the correct option is D.