Rate of diffusion is the change in number of diffusing molecules inside the cell over a time. Since, net movement of diffusion depends on the concentration gradient, the rate of diffusion ∝ concentration gradient across the membrane. Also, the ratio of rate of diffusion is inversely proportional to the square root of the masses. ∵ Mass of SO2=64 Mass of CO2=44 Mass of SO3=80 Mass of PCl3=137 ∴ Rate of diffusion of SO2,CO2,PCl3 and SO3, is CO2>SO2>SO3>PCl3 Hence, option (d) is correct.