**Core Concept**
Pepsinogen is a zymogen that plays a crucial role in protein digestion in the stomach. It is converted into its active form, pepsin, through a process of activation, allowing for the breakdown of proteins into smaller peptides and amino acids.

**Why the Correct Answer is Right**
Pepsinogen is activated by gastric acid, specifically the low pH environment of the stomach, which triggers a conformational change in the zymogen. This change exposes the active site of the enzyme, allowing it to cleave peptide bonds and initiate protein digestion. The activation of pepsinogen is an essential step in the digestive process, as it enables the stomach to break down proteins into smaller molecules that can be further digested and absorbed in the small intestine.

**Why Each Wrong Option is Incorrect**
**Option A:** This option is incorrect because pepsinogen is not activated by gastric amylase, which is involved in carbohydrate digestion, not protein digestion.
**Option B:** This option is incorrect because trypsinogen, not pepsinogen, is activated by enterokinase in the small intestine, and it plays a role in protein digestion in the small intestine, not the stomach.
**Option C:** This option is incorrect because chymotrypsinogen is a zymogen that is activated by trypsin in the small intestine, and it plays a role in protein digestion in the small intestine, not the stomach.

**Clinical Pearl / High-Yield Fact**
It's essential to remember that the activation of pepsinogen is pH-dependent, and the low pH environment of the stomach is necessary for its activation. This is why antacids and acid-reducing medications can impair protein digestion by reducing the activation of pepsinogen.

**Correct Answer: A. Gastric acid.**