Concept:Oxidation is defined as the gain of oxygen by a substance during a chemical reaction.Explanation:The given reaction is:CuO(s)+H2(g)→Cu(s)+H2O(l)In this reaction, CuO loses its oxygen atom to form copper metal, Cu.Because CuO loses oxygen, it undergoes reduction.Meanwhile, hydrogen gas, H2, gains that oxygen atom to form water, H2O.Since H2 gains oxygen, it undergoes oxidation.Therefore, the substance that is oxidized in this reaction is hydrogen gas.Answer:C. H2