## Core Concept
The question tests knowledge of H2 receptor antagonists, a class of drugs that reduce gastric acid secretion by blocking histamine H2 receptors on parietal cells in the stomach.

## Why the Correct Answer is Right
Famotidine, Nizatidine, and Ranitidine are all H2 receptor antagonists. is not an H2 blocker; it is a proton pump inhibitor (PPI), which works by irreversibly inhibiting the H+/K+ ATPase enzyme system at the secretory surface of gastric parietal cells.

## Why Each Wrong Option is Incorrect
* **Option A:** Famotidine is an H2 blocker.
* **Option B:** Nizatidine is an H2 blocker.
* **Option D:** Ranitidine is an H2 blocker.

## Clinical Pearl / High-Yield Fact
A key difference between H2 blockers and PPIs like is their onset and duration of action, as well as their effect on gastric pH. H2 blockers provide quicker relief but have a shorter duration of action compared to PPIs.

**Correct Answer: C. **