Why is diffusion alone not enough in multicellular organisms?

Correct answer: Because all cells are not in direct contact with the outside environment

Diffusion is effective only over short distances and depends on a concentration gradient. A multicellular organism has many layers of cells, so inner cells are far from the external environment and may have high oxygen and nutrient demands. Diffusion alone would be too slow to supply them and remove wastes efficiently. Hence option A is correct; the other choices are false generalisations about multicellular organisms.