## **Core Concept**
The question tests the knowledge of enzyme stability at different pH levels, specifically acidic pH. Enzymes are biological catalysts with optimal activity at specific pH ranges, and their stability can vary significantly across different pH levels. This property is crucial for their function in various physiological and pathological conditions.

## **Why the Correct Answer is Right**
The correct answer, **Pepsin**, is stable at acidic pH. Pepsin is a gastric enzyme that works optimally in the acidic environment of the stomach, where the pH is low (around 1.5-3.5). It is secreted by chief cells in the stomach as pepsinogen, which is then activated to pepsin by hydrochloric acid (HCl) in the stomach lumen. Pepsin's stability and activity at acidic pH allow it to break down proteins into smaller peptides during digestion.

## **Why Each Wrong Option is Incorrect**
- **Option A:** Trypsin is not stable at acidic pH; it is a pancreatic enzyme that works optimally at alkaline pH (around 8) in the small intestine. It is activated from trypsinogen by enterokinase in the intestinal lumen.
- **Option B:** Amylase, particularly salivary or pancreatic amylase, works best at neutral to slightly alkaline pH, not at acidic pH. It breaks down starches into sugars in the mouth and small intestine.
- **Option C:** This option seems to be missing, but assuming it refers to another enzyme not specified, the focus remains on why the mentioned options are incorrect.

## **Clinical Pearl / High-Yield Fact**
A key point to remember is that **pepsin** is the primary enzyme responsible for protein digestion in the stomach, and its activity is enhanced by the acidic environment. This acidic environment is maintained by the parietal cells' secretion of hydrochloric acid (HCl), which also activates pepsinogen to pepsin.

**Correct Answer: C. Pepsin.**