Antacids are basic species that are used as drugs to decrease stomach acidity. Sodium hydroxide is basic but it is too strong. It can cause health problems and thus can't be used as an antacid. Urea being an excretion product is not used as an antacid. Sodium bicarbonate and magnesium hydroxides are alkaline substances that can be used to increase stomach pH. Thus they are used as antacids.To treat heartburn, indigestion, or an upset stomach, people use antacids, which neutralise stomach acid. In those with acid reflux, the natural mucus barrier protecting the stomach lining can cause injury to the oesophagus when an excessive quantity of acids are created in the stomach. The chemical neutralisation of stomach acid by alkaline ions in antacids prevents damage and relieves discomfort.

Hence, option b and c is correct.