**Core Concept**
The question requires identification of a gram-positive cocobacillus exhibiting characteristic tumbling motility in cerebrospinal fluid (CSF), which is indicative of meningitis. This organism is a type of bacteria that has a unique morphology and motility pattern.

**Why the Correct Answer is Right**
Listeria monocytogenes is a gram-positive, rod-shaped bacterium that exhibits tumbling or end-over-end motility at room temperature. This motility pattern is characteristic and can be observed in CSF in cases of listerial meningitis. Listeria monocytogenes is a common cause of meningitis, particularly in immunocompromised individuals, the elderly, and pregnant women. The organism invades the central nervous system through the bloodstream and can cause severe inflammation of the meninges.

**Why Each Wrong Option is Incorrect**
**Option B:** Neisseria meningitidis (Meningococcus) is a gram-negative diplococcus that can cause meningitis, but it does not exhibit tumbling motility. It is typically seen as pairs of cocci in Gram stain.
**Option C:** Haemophilus influenzae is a gram-negative coccobacillus that can cause meningitis, particularly in children. However, it does not exhibit tumbling motility and is typically associated with a more severe clinical presentation.
**Option D:** Streptococcus pneumoniae (Pneumococcus) is a gram-positive diplococcus that can cause meningitis, but it does not exhibit tumbling motility. It is typically seen as pairs of cocci in Gram stain.

**Clinical Pearl / High-Yield Fact**
Listeria monocytogenes is a zoonotic pathogen, meaning it can be transmitted from animals to humans. It is commonly found in contaminated dairy products, meat, and produce, highlighting the importance of proper food handling and hygiene in preventing listerial infections.

✓ Correct Answer: A. Listeria