**Core Concept**
The question is testing the ability to identify anti-tuberculous drugs that can effectively penetrate the cerebrospinal fluid (CSF), which is crucial for treating tuberculous meningitis. This requires knowledge of the pharmacokinetic properties of these drugs, particularly their ability to cross the blood-brain barrier.

**Why the Correct Answer is Right**
Ethambutol is the correct answer because it has excellent penetration into the CSF, making it effective against tuberculous meningitis. This is due to its ability to cross the blood-brain barrier, which is facilitated by its lipid solubility and low protein binding. Ethambutol's penetration into the CSF is essential for treating central nervous system (CNS) tuberculosis, including tuberculous meningitis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Isoniazid has limited penetration into the CSF, and its effectiveness against tuberculous meningitis is reduced due to its low CSF concentration.
* **Option B:** Rifampicin has moderate penetration into the CSF, but it is not as effective as ethambutol in treating CNS tuberculosis.
* **Option D:** Pyrazinamide has poor penetration into the CSF, making it less effective against tuberculous meningitis.

**Clinical Pearl / High-Yield Fact**
It is essential to note that the effectiveness of anti-tuberculous drugs in the CSF is crucial for treating tuberculous meningitis, and ethambutol's excellent penetration into the CSF makes it a critical component of CNS tuberculosis treatment regimens.

**Correct Answer:** C. Ethambutol.