Hello!
In this chemical equation, there are two main elements. Those elements are magnesium (Mg) and oxygen (O).
Molecules are a group of atoms bonded together, and in this case, the oxygen molecules is O2, but not 2MgO because there are two different molecules in this compound.
Coefficients are a number that multiply an entire molecule, and are also placed in front of it. In this case, O2 has no coefficient, but a subscript of 2. But, every molecule in this equation must have a coefficient and if add a coefficient of 1 to the oxygen molecule, it does nothing to the overall balancing of equations.
Therefore, the coefficient to the oxygen molecule is choice C, 1.
