Which substance is commonly used as an antacid to neutralise excess acid in the stomach?

Correct answer: Sodium hydrogen carbonate

An antacid works through acid-base neutralisation, so it must be a mild base that can react with excess hydrochloric acid. Sodium hydrogen carbonate, NaHCO₃, is a commonly taught antacid and reacts with HCl to produce salt, water, and carbon dioxide. Hence option B is the intended answer. Calcium carbonate can also neutralise acid, but the question’s standard expected example is sodium hydrogen carbonate.