**Core Concept**
The question is testing the knowledge of opportunistic infections in immunocompromised patients, specifically those with AIDS. The correct answer requires understanding the pathophysiology of meningitis in immunocompromised individuals and the common causative organisms.

**Why the Correct Answer is Right**
In AIDS patients, the immune system is severely compromised, making them susceptible to opportunistic infections. Amongst the common causative organisms of acute meningitis in AIDS patients, Cryptococcus neoformans is the most prevalent. This organism is a type of fungus that typically infects the lungs but can spread to the central nervous system in immunocompromised individuals. The pathogenesis involves the inhalation of spores, which then germinate into yeast cells that penetrate the blood-brain barrier and cause meningitis.

**Why Each Wrong Option is Incorrect**
* **Option A:** Although Listeria monocytogenes is a cause of meningitis, especially in neonates and the elderly, it is not the most common organism in AIDS patients.
* **Option B:** Escherichia coli is a common cause of meningitis in neonates but not typically in AIDS patients.
* **Option D:** Haemophilus influenzae type b was a common cause of meningitis in children before the introduction of Hib vaccine but is not the most common organism in AIDS patients.

**Clinical Pearl / High-Yield Fact**
Cryptococcal meningitis is a classic opportunistic infection in AIDS patients and requires immediate antifungal therapy, including amphotericin B and fluconazole.

**Correct Answer:** C. Cryptococcus neoformans.