Diffusion is insufficient to meet ______ requirement of multicellular organisms.

The correct option is A Oxygen
Diffusion is insufficient to meet oxygen requirement of multicellular organisms because in multicellular organisms all the cells can not come into contact of air and thus diffusion is not possible for each cell and to fulfill the oxygen need of the organism also. In unicellular organisms, it is possible very easily. Plants are multicellular but they have large intercellular spaces where air passes, so it is possible for them to fulfill the oxygen need by diffusion.

Therefore, the correct answer is option A.