**Core Concept**
The question is testing the knowledge of chemoprophylaxis in a specific scenario. Chemoprophylaxis is the administration of medication to prevent disease in individuals who are at high risk of developing the disease, but are not yet symptomatic. In the context of tuberculosis (TB), chemoprophylaxis is recommended for close contacts of individuals with active TB disease, especially in children.

**Why the Correct Answer is Right**
The correct answer is Isoniazid (INH). INH is the primary medication used for chemoprophylaxis in individuals at high risk of developing TB disease. It is effective in preventing the development of TB disease in close contacts, including children. INH works by inhibiting the growth of Mycobacterium tuberculosis, the bacterium that causes TB. The recommended duration of INH chemoprophylaxis is 6-9 months in children.

**Why Each Wrong Option is Incorrect**
**Option A:** Rifampicin is not typically used as a chemoprophylaxis agent in children due to concerns about resistance and side effects.
**Option B:** Pyrazinamide is not effective as a monotherapy for chemoprophylaxis and is usually used in combination with INH for treatment of active TB.
**Option C:** Ethambutol is not typically used as a chemoprophylaxis agent in children due to its potential for ototoxicity and other side effects.

**Clinical Pearl / High-Yield Fact**
It's essential to note that chemoprophylaxis is only effective in preventing the development of TB disease if the individual is not already infected with M. tuberculosis. Therefore, a tuberculin skin test (TST) or interferon-gamma release assay (IGRA) should be performed before initiating chemoprophylaxis to rule out latent TB infection.

**Correct Answer:** I. Isoniazid.