**Core Concept**
The question requires knowledge of the pH stability of various enzymes. Enzymes are proteins that catalyze biochemical reactions and often have specific optimal pH ranges for their activity. Some enzymes are stable and functional at acidic pH, while others are denatured or inactivated.

**Why the Correct Answer is Right**
The correct answer is an enzyme that is stable at acidic pH, such as pepsin. Pepsin is a gastric enzyme that breaks down proteins into smaller peptides. It is secreted by the chief cells of the stomach lining and is activated by the acidic environment of the stomach, where the pH is around 1.5-3.5. Pepsin's stability at acidic pH allows it to effectively digest proteins in the stomach. The optimal pH for pepsin activity is around pH 2, which is acidic.

**Why Each Wrong Option is Incorrect**
* **Option A:** This option is incorrect because it is not a specific enzyme. Without knowing the name of the enzyme, it is impossible to determine its pH stability.
* **Option B:** This option is incorrect because the enzyme is not known for its stability at acidic pH. Most enzymes are denatured or inactivated at acidic pH, which would prevent them from functioning properly.
* **Option C:** This option is incorrect because the enzyme is not stable at acidic pH. The enzyme is likely to be denatured or inactivated at acidic pH, which would prevent it from functioning properly.

**Clinical Pearl / High-Yield Fact**
It is essential to remember that enzymes have specific optimal pH ranges for their activity. Knowing the pH stability of enzymes can help you understand their function and behavior in different environments, such as the acidic environment of the stomach.

**Correct Answer: D. Pepsin**