**Core Concept**
The question is testing the student's knowledge of antimicrobial therapy, specifically the treatment of extracellular mycobacteria. Extracellular mycobacteria, such as M. tuberculosis, require effective antibiotics to inhibit their growth and proliferation.

**Why the Correct Answer is Right**
Isoniazid (INH) is a first-line antitubercular agent that is most effective against extracellular mycobacteria. INH acts by inhibiting the synthesis of mycolic acid, a critical component of the mycobacterial cell wall, through the inhibition of the enzyme enoyl-ACP reductase (InhA). This leads to the disruption of the bacterial cell wall and ultimately, the death of the bacteria.

**Why Each Wrong Option is Incorrect**
**Option A:** Streptomycin is an aminoglycoside antibiotic that is effective against intracellular mycobacteria, not extracellular mycobacteria.
**Option B:** Ethambutol is an antitubercular agent that inhibits the synthesis of arabinogalactan, a component of the mycobacterial cell wall. However, it is less effective than INH against extracellular mycobacteria.
**Option C:** Rifampicin is a powerful antitubercular agent that inhibits RNA synthesis by binding to the beta-subunit of RNA polymerase. While it is effective against extracellular mycobacteria, it is not the most effective drug against this target.

**Clinical Pearl / High-Yield Fact**
It is essential to note that INH and rifampicin are often used in combination to treat tuberculosis, as this increases the efficacy of treatment and reduces the risk of resistance development.

**Correct Answer: D. Isoniazid (INH)**