The answer is B.
in the original equation, CH4 + O2 → CO2 + H2O, there are:
REACTANTS: 2 carbon, 4 hydrogen, 2 oxygen
PRODUCTS: 2 carbon, 2 hydrogen and 1 oxygen.
If we change the equation to this,
CH4 + 2O2 → CO2 + 2H2O, then there are now:
REACTANTS: 2 carbon, 4 hydrogen, 4 oxygen
PRODUCTS: 2 carbon, 4 hydrogen, 4 oxygen
and thus the equation is balanced
