Concept:Magnesium hydroxide is a mild base that neutralizes excess stomach acid, so it is used as an antacid.Explanation:Antacids work by neutralizing excess hydrochloric acid in the stomach.Magnesium hydroxide, known as milk of magnesia, is a weak base that reacts with acid to form salt and water.It is not an oxidizing agent (gains electrons) or a reducing agent (loses electrons).It is also not a fermentation agent (which uses microbes to convert carbohydrates).Thus, it acts as an anti-acid to relieve acidity.Answer:C. anti-acid