The term oxidation, when focusing on Oxygen, it will defined as the adding or removing of Oxygen to a compound. Oxidation will be the gain of Oxygen into forming a new compound and Reduction will be the loss of Oxygen. For our question the answer that best follows this explanation is letter C, if I am turning a metal into a metallic oxide, I am using the process of oxidation, as we can see by some examples of metallic oxides, MgO2, CaO2, Cu2O2.