Why is diffusion alone not sufficient in multicellular organisms?

Correct answer: Because all cells are not very close to the external environment

Diffusion is effective only across short distances and depends on a concentration gradient. In a multicellular organism, many cells lie deep inside the body and are far from the external environment or the surface where gases and nutrients enter. Diffusion alone would be too slow to meet their continuous demands or remove wastes efficiently, so specialized transport systems are needed. Therefore option A is correct; the other statements contradict basic biology.