Option 1, Trypsin, is a digestive enzyme that breaks down proteins in the small intestine. Option 3, Ptyalin, is another name for salivary amylase, which initiates the digestion of carbohydrates in the mouth. Option 4, Pepsin, is a stomach enzyme that breaks down proteins into peptides.

Option 2, Gastrin, however, is not a digestive enzyme. It`s a hormone produced by the stomach and duodenum. Its role is to stimulate the production of gastric acid, which helps digest and break down food, but it doesn`t break down substances itself, hence it`s not an enzyme. So, the correct answer is 2, Gastrin.

It`s essential to know the difference between enzymes and hormones as they play different roles in the body, despite both being involved in the digestive process.