**Core Concept**
The question is testing the student's ability to identify the causative agent of meningitis based on the Gram stain characteristics of the cerebrospinal fluid (CSF). The student needs to recall that gram-positive bacilli are often associated with certain bacterial pathogens.

**Why the Correct Answer is Right**
The correct answer is **Listeria monocytogenes**. This bacterium is a gram-positive rod that can cause meningitis, particularly in neonates, immunocompromised individuals, and pregnant women. Listeria monocytogenes can invade the central nervous system and cause inflammation of the meninges, resulting in symptoms such as fever, headache, and stiff neck. The bacterium can also cause septicemia, which can be life-threatening.

**Why Each Wrong Option is Incorrect**
* **Option A:** Escherichia coli is a gram-negative rod that can cause meningitis, but it is not the most likely causative agent in this scenario.
* **Option B:** Streptococcus pneumoniae is a gram-positive diplococcus that can cause meningitis, but it is not a gram-positive bacillus.
* **Option D:** Haemophilus influenzae is a gram-negative rod that can cause meningitis, but it is not a gram-positive bacillus.

**Clinical Pearl / High-Yield Fact**
Listeria monocytogenes is a zoonotic pathogen that can be transmitted from animals to humans through contaminated food and water. Pregnant women, immunocompromised individuals, and neonates are at increased risk of developing listeriosis, which can lead to meningitis and septicemia.

**Correct Answer:** C. Listeria monocytogenes