An antacid taken for excess stomach acid works on which principle?

Correct answer: Neutralisation

Excess hydrochloric acid in the stomach can cause irritation and a burning sensation. An antacid contains a mild base, such as a suitable hydroxide, carbonate, or bicarbonate, which reacts with the excess acid. The acid-base reaction reduces acidity and forms salt, water, and sometimes carbon dioxide. This process is called neutralisation, so option C is correct; the other choices are physical processes or unrelated effects.