Which factor would NOT increase the rate of diffusion?

Increased thickness of the membrane would not increase the rate of diffusion. In fact, a thicker membrane generally serves as a barrier to diffusion, making it more difficult for substances to pass through. This is because the distance that a molecule must travel to diffuse from one side of the membrane to the other increases with membrane thickness, thereby reducing the rate at which the molecules can move across.

On the other hand, the other factors mentioned facilitate the process of diffusion. Increased surface area allows more molecules to pass through the membrane simultaneously, higher temperatures provide molecules with greater kinetic energy, increasing their movement, and increased permeability of the membrane allows substances to pass through more readily. Therefore, while the other factors enhance diffusion, a thicker membrane acts to impede it, making it the correct choice for this question.