Antacids are medications used to neutralize excess acid in the stomach, providing relief from conditions like heartburn and acid indigestion. The correct answer to this question is option 2, magnesium hydroxide.

Option 1, sodium hydrogen phthalate, is not commonly used as an antacid. It is a chemical compound commonly used as a primary standard in acid-base titrations.

Option 3, calcium hydroxide, is also used as an antacid, but it is less commonly used compared to magnesium hydroxide. Calcium hydroxide is often used in combination with magnesium hydroxide or aluminum hydroxide to provide more effective relief from acidity.

Option 4, manganese acetate, is not used as an antacid. Manganese acetate is a chemical compound often used as a catalyst or reagent in organic synthesis.

Overall, the most commonly used antacid among the options provided is magnesium hydroxide. It works by neutralizing excess stomach acid, but it may have side effects like diarrhea. It is important to consult a healthcare professional before using any medication, including antacids, to ensure appropriate use and dosage.