Concept:According to Graham's law, the rate of diffusion of a gas is inversely proportional to the square root of its molar mass.Therefore, the gas with the smallest molar mass diffuses the fastest.Explanation:Calculate the molar mass of each gas.A. Cl2=2×35.5=71.0B. SO2=32.0+2(16.0)=64.0C. CH4=12.0+4(1.0)=16.0D. C2H6=2(12.0)+6(1.0)=30.0The lowest molar mass is 16.0, which belongs to CH4.Since lighter gases diffuse more rapidly, CH4 will diffuse most quickly.Answer:C. CH4