Captopril is an ACE (Angiotensin-Converting Enzyme) inhibitor, a class of drugs primarily used to treat hypertension (high blood pressure) and heart failure. It works by blocking the conversion of angiotensin I to angiotensin II, a potent vasoconstrictor, thereby lowering blood pressure. Options B (acidity) and C (arthritis) are incorrect - Captopril has no role in treating these conditions.