Which one of the following compounds is present in antacid tablets or medicines which are used to cure indigestion and acidity in the stomach ?

Antacids are mild bases used to neutralize excess hydrochloric acid (HCl) produced in the stomach, which causes indigestion and acidity. Aluminium hydroxide (Al(OH)₃) is a common active ingredient in many over-the-counter antacid tablets and liquids. It reacts with stomach acid to form aluminium chloride and water, thereby raising the pH level.

Options A (Sodium hydroxide) and B (Potassium hydroxide) are strong bases that are highly corrosive and caustic; they would cause severe chemical burns to the esophagus and stomach lining if ingested. Option C (Lithium hydroxide) is also a strong, corrosive base and is not used medicinally for acidity. Other common antacids include magnesium hydroxide (Milk of Magnesia), calcium carbonate, and sodium bicarbonate (baking soda).