By Fick's first law of diffusion, rate of simple diffusion is NOT propoional to:

Fick's first law for diffusion: Rate of simple diffusion = - D.A X /d Minus sign indicates downhill transpo D - Diffusion coefficient A - surface area available for diffusion C1 - C2 is the concentration gradient of the diffusing substance d - diffusion distance or thickness of the membrane through which the substance is diffusing. Simple diffusion occurs by viue of kinetic energy of the molecules, and hence is directly propoional to the temperature. However, in the case of Fick's law for describing the rate of simple diffusion, temperature is held constant (same) on either side of the membrane. Hence, in the context of this MCQ, the answer is temperature.