**Core Concept**
Antitubercular drugs work through various mechanisms to inhibit Mycobacterium tuberculosis growth. Bacteriostatic agents, like some antibiotics, prevent bacterial multiplication without necessarily killing the bacteria.

**Why the Correct Answer is Right**
The correct answer is Isoniazid (H) because it is primarily bacteriostatic against Mycobacterium tuberculosis. Isoniazid exerts its effect by inhibiting the synthesis of mycolic acid, an essential component of the mycobacterial cell wall. This disruption prevents the bacteria from multiplying and forming new cell walls, ultimately leading to bacteriostasis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Rifampicin is bactericidal, as it rapidly kills Mycobacterium tuberculosis by inhibiting RNA synthesis.
* **Option B:** Ethambutol is bacteriostatic, but its primary mechanism of action is not as well-established as Isoniazid's. Ethambutol may inhibit arabinosyl transferases, enzymes involved in cell wall synthesis.
* **Option C:** Pyrazinamide is bactericidal, particularly effective against intracellular M. tuberculosis in acidic environments, such as the macrophage phagosome.

**Clinical Pearl / High-Yield Fact**
Isoniazid and Rifampicin are the backbone of tuberculosis treatment, with Isoniazid being bacteriostatic and Rifampicin being bactericidal. This fundamental principle is crucial for understanding the mechanisms of action of these antitubercular drugs.

**Correct Answer:** H. Isoniazid.