Image (a) will have a greater rate of diffusion compared to (b) and (c). Molecules in a tend to move from a region where they are highly concentrated( upper side of the plasma membrane) to the cytoplasm where they are less concentrated. Molecules move down their concentration gradient by crossing the lipid portion of the membrane directly while other molecules pass by facilitated diffusion. The ampipathy of plasma membrane allows movement of the molecules selectively. The process is aided by a concentration gradient which equalizes the molecules.
Molecules tend to move through the cell's cytosol by diffusion while some molecules diffuse across the plasma membrane. Each substance has a concentration gradient which is independent of the other. A larger concentration gradient in (a) results into faster diffusion.