According to the chemical equation, two moles of O2 react with 1 mole of CH4, so the ratio is 2:1. This means if the reaction takes 12.0 moles of CH4, it would react with 24.0 moles of O2. As you can see, we just have 20.0 moles of O2, which means the limiting reactant is the oxygen because CH4 would have a reamining after the reaction.
Therefore, the answer is a. O2.
