Achemical equation is balanced when the
a. coefficients of the reactants equal the coefficients of the products.
b. same number of each kind of atom appears in the reactants and in the products.
c. products and reactants are the same chemicals.
d. subscripts of the reactants equal the subscripts of the products.

Answer is B. The law of conservation of mass states that mass can't be destroyed or created in a chemical reaction. The atoms of each element make up this mass. The elements have to have the same number of atoms for each element on both sides. If you have 2 carbon atoms and one oxygen, but your product has 2 carbon atoms and 2 oxygen, it is unbalanced. The only reason D isn't the answer is because you can have different subscripts and still have a balanced equation. It all depends on your coefficients