Achemical equation is balanced by changing or adding
a.
chemical symbols.
c.
numbers in front of molecules.
b.
subscripts after atoms.
d.
reactants in the product.

Number infront of molecules are the no of moles and they can be varied.
A chemical equation is balanced by changing or adding numbers in front of molecules.
So , c.
numbers in front of molecules. is the correct answer.

It will appear as o2(g) as a reactant