For a chemical equation to be balanced, the number of atoms of each element on the left Hans side of the reaction equation must equal the number of atoms of that element on the right hand side of the reaction equation.
Atoms of oxygen are six on both sides of the reaction equation. Atoms of sulphur are two while there is only one atom of carbon on both sides of the reaction equation.
To consume the 2.8 moles of CH4 we need 5.6 moles of O2 since the molar ratio is 1:2. We have only 3 moles of O2 ; therefore, O2 is the limiting reactant.