What needs to happen for the equation ch4 + o2 →  co2 + h2o to be balanced? a. a coefficient of 2 must be added before both products. b. a coefficient of 2 must be added before the second reactant and the second product. c. nothing, the equation is already balanced. d. a coefficient of 2 must be added before both products and reactants.

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