Answer:
Option (d).
Explanation:
Simple diffusion can be defined as a transport method, which involves movement of non-polar, uncharged substances across the lipid bilayer without help of membrane proteins.
Simple diffusion is unsaturated, unlike facilitated diffusion. This is because membrane proteins are not required for simple diffusion, while during facilitated diffusion, membrane proteins become saturated at maximum rate of diffusion.
Thus, the correct answer is option (d).
