The correct answer is option C, activation of pepsinogen to pepsin.

Gastric parietal cells are specialized cells found in the lining of the stomach. One of their main functions is to secrete hydrochloric acid (HCl) into the stomach. This secretion of HCl plays a crucial role in the digestion process. Let's understand why the activation of pepsinogen to pepsin is dependent on the secretion of HCl.

1. Introduction:
Gastric parietal cells secrete HCl, which serves multiple functions in the stomach. One of these functions is the activation of pepsinogen to pepsin.

2. Pepsinogen:
Pepsinogen is an inactive precursor enzyme produced by chief cells in the stomach. It is released into the gastric juice and does not have any proteolytic activity.

3. Activation of Pepsinogen:
HCl plays a vital role in converting pepsinogen into its active form, pepsin. The acidic environment created by HCl in the stomach stimulates the conversion of pepsinogen to pepsin.

4. Optimal pH for Pepsin Activity:
Pepsin is a proteolytic enzyme that breaks down proteins into smaller peptides. It functions optimally at a pH of around 1.5-2, which is highly acidic. The secretion of HCl by gastric parietal cells establishes the required acidic environment for pepsin activity.

5. Mechanism of Activation:
HCl activates pepsinogen by removing a part of its molecule, resulting in the formation of pepsin. This activated form of the enzyme has proteolytic activity and can begin the digestion of proteins.

6. Importance of Pepsin Activation:
The hydrolysis of proteins into smaller peptides by pepsin is an essential step in protein digestion. These smaller peptides can be further broken down by other enzymes, such as pancreatic proteases, into individual amino acids for absorption.

7. Other Functions of HCl:
Apart from activating pepsinogen, HCl has other important functions in the stomach. It helps in the sterilization of ingested food by killing most bacteria and other microorganisms. It also denatures proteins, making them more accessible to enzymatic digestion.

In conclusion, the secretion of HCl by gastric parietal cells is necessary for the activation of pepsinogen to pepsin. This activation step is crucial for proper protein digestion in the stomach.