Answer is: C. Reactants combine to form products.
In a chemical reaction, chemical bonds are forming and breaking and this cause the transformation of one group of chemical substances to another
For example synthesis of water from its elements: 2H₂ + O₂ → 2H₂O, nonpolar bonds between hydrogen and oxygen atoms (reactants) are breaking and new polar bonds are forming between hydrogen and oxygen in molecule of water (product).
