Concept:
Many pharmaceutical preparations have common (trade-like) names that hide a simple chemical formula. "Milk of Magnesia" is one such classic preparation.
Step 1: Milk of Magnesia is an aqueous suspension of magnesium hydroxide, Mg(OH)2, which gives it its characteristic milky white appearance.
Step 2: It is used both as an antacid (it neutralises stomach acid) and, in larger doses, as an osmotic laxative.
Why the others are wrong: Magnesium carbonate and magnesium trisilicate are other antacids but are not Milk of Magnesia. Magnesium sulphate (Epsom salt) is a saline purgative. The specific compound in Milk of Magnesia is magnesium hydroxide.
Answer: Option (2) — Magnesium Hydroxide.