**Core Concept**
The question is testing the concept of selective agents used in microbiological media, specifically in Lowenstein-Jensen (LJ) medium for culturing Mycobacteria. Lowenstein-Jensen medium is a solid medium used for the isolation and growth of Mycobacterium species, and it requires a selective agent to inhibit the growth of unwanted bacteria.

**Why the Correct Answer is Right**
The selective agent for Mycobacteria in LJ medium is Isoniazid (INH). Isoniazid inhibits the growth of most non-tuberculous bacteria, allowing the Mycobacterium tuberculosis to grow. This is because INH is a prodrug that requires activation by the catalase-peroxidase enzyme (KatG) found in M. tuberculosis. The activated form of INH then inhibits the synthesis of mycolic acid, a key component of the Mycobacterium cell wall. This unique mechanism of action makes INH an effective selective agent for M. tuberculosis in LJ medium.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because it is not a selective agent for Mycobacteria in LJ medium.
**Option B:** This option is incorrect because it is not a selective agent for Mycobacteria in LJ medium.
**Option D:** This option is incorrect because it is not a selective agent for Mycobacteria in LJ medium.

**Clinical Pearl / High-Yield Fact**
Remember that Isoniazid is not only a selective agent in LJ medium but also a first-line antitubercular medication used in the treatment of tuberculosis. It is essential to note that INH resistance is a significant concern in tuberculosis treatment, and its use should be guided by susceptibility testing.

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