**Core Concept**
Meningococcal infection is caused by the bacterium *Neisseria meningitidis*. Selective media are used to isolate and cultivate specific bacteria while inhibiting the growth of other microorganisms.

**Why the Correct Answer is Right**
The correct selective medium for *Neisseria meningitidis* is Thayer-Martin agar, which is a type of chocolate agar enriched with antibiotics to inhibit the growth of other bacteria. Thayer-Martin agar contains antibiotics such as vancomycin, nystatin, colistin, and trimethoprim, which selectively inhibit the growth of other bacteria while allowing *Neisseria meningitidis* to grow. This is because *Neisseria meningitidis* is resistant to these antibiotics.

**Why Each Wrong Option is Incorrect**
* **Option A:** Blood agar is a general-purpose medium that supports the growth of many types of bacteria, but it is not selective for *Neisseria meningitidis*.
* **Option B:** MacConkey agar is a selective medium for gram-negative bacteria, but it is not specific for *Neisseria meningitidis*.
* **Option D:** Sabouraud agar is a medium used for cultivating fungi, and it is not suitable for *Neisseria meningitidis*.

**Clinical Pearl / High-Yield Fact**
*Neisseria meningitidis* is a major cause of bacterial meningitis, and early diagnosis is critical for treatment. Thayer-Martin agar is an essential tool in the laboratory diagnosis of meningococcal infection.

**Correct Answer: C. Thayer-Martin agar**