**Core Concept**
The question targets the identification of the causative agent of the most fatal form of amoebic encephalitis, a rare but severe infection of the brain. This condition is characterized by inflammation of the brain tissue due to the invasion of amoebic pathogens.

**Why the Correct Answer is Right**
Naegleria fowleri, a thermophilic amoeba, is the primary cause of primary amebic meningoencephalitis (PAM), a nearly always fatal disease. This amoeba invades the brain through the olfactory nerve after entering the nasal cavity, usually through contaminated water. The amoeba feeds on brain tissue, causing rapid destruction of the central nervous system. Naegleria fowleri thrives in warm and alkaline environments, which explains its prevalence in freshwater lakes and hot springs.

**Why Each Wrong Option is Incorrect**
**Option A:** E. histolytica is the causative agent of amoebic dysentery, a gastrointestinal infection that does not typically lead to encephalitis. While it's an amoeba, it's not associated with brain infections.

**Option C:** E. dispar is a non-pathogenic amoeba closely related to E. histolytica, but it doesn't cause human infections. It's often used as a reference strain in research.

**Option D:** Acanthamoeba species can cause granulomatous amoebic encephalitis (GAE), a different form of amoebic encephalitis, typically in immunocompromised individuals. However, GAE is less aggressive and less commonly fatal than PAM caused by Naegleria fowleri.

**Clinical Pearl / High-Yield Fact**
Naegleria fowleri is often associated with swimming in warm freshwater lakes, hot springs, or contaminated waterways during the summer months. Rapid diagnosis and treatment with intravenous amphotericin B are critical for survival, emphasizing the importance of prompt medical evaluation in patients presenting with sudden-onset headache, fever, and altered mental status following exposure to contaminated water.

**✓ Correct Answer: B. Naeglaria**