Question

why is diffusion insufficient to meet the oxygen requirement of multicellular organisms like humans?​

Answer 1

Diffusion, being a slow process will take a lot of time to circulate oxygen to all the body cells. Hence, diffusion is insufficient to meet the oxygen requirements of multicellular organisms like humans.so this is the answer

Answer 2

here is the answer of your question ...multicellular organisms have Complex body designs and a large body shape.t

hence they need more oxygen to carryout cellular respiration.unlike unicellular organisms, multicellular organisms do not have all the cells of the body in direct contact with the environment .hence diffusion cannot meet the energy requirements of our body needs.
hope this will help you