The correct answer for the protein-digesting enzyme secreted by the stomach wall in mammals is option 3, pepsin.

Option 1, chitinase, is not the correct answer because chitinase is an enzyme that breaks down chitin, a complex carbohydrate found in the exoskeletons of arthropods and cell walls of fungi. It is not involved in digesting proteins.

Option 2, amylase, is also not the correct answer. Amylase is an enzyme that breaks down carbohydrates, specifically starch, into simpler sugars. It is primarily produced in the salivary glands and pancreas, not in the stomach.

Option 4, trypsin, is a protein-digesting enzyme, but it is not secreted by the stomach wall. Trypsin is produced in the pancreas and is released into the small intestine to aid in protein digestion.

Pepsin, on the other hand, is the correct answer. It is a proteolytic enzyme secreted by the gastric glands in the stomach wall. Pepsin is activated by the acidic environment of the stomach and plays a crucial role in breaking down proteins into smaller peptides for further digestion and absorption in the small intestine.