**Core Concept**
The question is testing the hepatotoxicity profile of various anti-tubercular drugs, which is crucial for managing patients with tuberculosis (TB) and minimizing liver damage. Hepatotoxicity is a significant side effect of many anti-TB medications, necessitating careful selection and monitoring.

**Why the Correct Answer is Right**
Ethambutol (B) is generally considered the least hepatotoxic among the first-line anti-tubercular drugs. This is because ethambutol's primary side effect is optic neuritis, which affects the eyes, rather than liver damage. Ethambutol works by inhibiting the synthesis of mycolic acid in Mycobacterium tuberculosis, ultimately disrupting the bacterial cell wall.

**Why Each Wrong Option is Incorrect**
**Option A:** Isoniazid (INH) is widely recognized for its hepatotoxic potential, with a risk of severe liver damage, especially in patients with pre-existing liver disease or those taking other hepatotoxic medications.

**Option C:** Pyrazinamide is another first-line anti-TB medication with significant hepatotoxicity, often leading to elevated liver enzymes and, in rare cases, severe liver injury.

**Clinical Pearl / High-Yield Fact**
When prescribing anti-TB medications, it's essential to regularly monitor liver function tests (LFTs) and adjust the treatment regimen as needed to minimize hepatotoxicity.

**Correct Answer:** B. Ethambutol.