CH4 will effuse the fastest. The speed of effusion is proportional to the speed of the gas molecules with faster molecules effusing fast than slower molecules. And the speed of the molecules at any given temperature is inversely related to the mass of the molecules with lighter molecules being faster. So let's take a look at the mass of the candidate molecules and see which is lighter and therefore faster. CH4 - Molar mass is about 16. CO - Molar mass is about 28 Ar - Molar mass is about 40 CO2 - Molar mass is about 44 If the 4 options, CH4 (methane) is by far the lightest, and therefore the fastest, so will effuse faster than the other 3 gasses.