Concept:
Adsorbents are substances that bind toxins, bacteria, and excess fluid onto their large surface area. In diarrhea, an adsorbent helps by soaking up irritants and giving more form to the stool.
Step 1: Light kaolin is a natural hydrated aluminum silicate clay. Its fine particles have a huge surface area that adsorbs toxins and excess water in the gut, which is exactly why it is the classic inorganic adsorbent used in antidiarrheal preparations (such as kaolin–pectin mixtures).
Step 2: The others have different uses — ferrous sulphate treats iron-deficiency anemia, calcium carbonate is an antacid and calcium supplement, and magnesium hydroxide is an antacid and laxative (it would actually worsen diarrhea). None act as the antidiarrheal adsorbent.
Answer: Option (3) — Light Kaolin is the inorganic adsorbent used in the treatment of diarrhea. (3)