In the stomach, gastric juice contains HCl which serves many functions:
1. It disinfects the food by killing bacteria.
2. It stops the action of salivary enzymes n due course of time.
3. It activates pepsinogen to pepsin and prorenin to rennin.
4. It produces an appropriate acidic pH for protein digestion.
5. It softens the food and breaks down the cement material between the cells of food.
6. It denatures many food proteins, increasing exposure of their peptide bonds to pepsin. This facilitates the action of pepsin on them.
Thus, the correct answer is option D.