Parietal cells are located in the body of the stomach, and are responsible for secreting acid when stimulated by a variety of hormones, including gastrin (from G cells) and histamine (from enterochromaffin-like (ECL) cells). The release of acid allows food particles to be broken down in the stomach before they are transported through the antrum to the small intestine.

Chief cells produce pepsinogen, trypsin is released from the pancreas into the small intestine, and ECL cells release histamine to stimulate parietal cells to secrete acid.

Enterochromaffin-like (ECL) cells are neuroendocrine cells in the body of the stomach that release histamine to stimulate the secretion of acid by parietal cells. They function in an indirect manner to decrease the pH of the stomach.

Chief cells release pepsinogen, which is activated to become pepsin by the acidic pH of the stomach and promotes protein breakdown. G cells release gastrin from the antrum of the stomach to encourage acid secretion by parietal cells.

Chief cells are responsible for secreting the zymogen pepsinogen into the lumen of the stomach. Once pepsinogen enters the acidic environment of the stomach, it auto-catalyzes its conversion into the fully functional enzyme pepsin, which serves to breakdown proteins into smaller amino acid units by cleaving peptide bonds. The reason that chief cells secrete the zymogen, rather than active pepsin, is to prevent the active enzyme from degrading the walls of the stomach.

Trypsin and trypsinogen are released from the pancreas and also help to break down proteins in the small intestine.

Parietal cells, in addition to secreting hydrochloric acid into the stomach lumen, also make intrinsic factor (IF) that serves to scavenge vitamin B-12 in the stomach and chaperone it to the ileum for absorption. IF is important because a lack of B-12 is implicated in numerous metabolic diseases and anemia.

The other enzymes listed are produced by different cells. Pepsinogen is released by chief cells of the stomach, and trypsin and amylase by the acinar cells of the pancreas.

Goblet cells secrete a thick layer of mucous to prevent autodigestion of the stomach wall by enzymes secreted into the lumen, such as pepsin, and the low pH of the stomach contents. D cells secrete somatostatin to prevent gastric acid secretion, G cells secrete gastrin to promote acid secretion into the gastric lumen, and enterochromaffin-like (ECL) cells secrete histamine to promote acidification of the gastric lumen.

G cells secrete the large peptide hormone gastrin, which travels through blood to stimulate parietal cells to secrete hydrochloric acid. 

Mucous cells produce mucus to protect the epithelial lining of the stomach from the harsh acidic environment. Chief cells are found in the exocrine glands and secrete pepsinogen, which is activated by the low pH of the stomach to become pepsin. Pepsin catalyzes the breakdown of protein into peptides. 

During digestion, the small intestine plays a major role in absorption and digestion of compounds. Because the contents of the stomach are extremely acidic, bicarbonate serves to neutralize hydrochloric acid, and raises the pH to an optimal level at which the enzymes can break down food.

Amylase is present in both saliva and the small intestine and breaks down starches and carbohydrates. Peptidase disrupts peptide bonds and digests proteins into amino acids. Lipase acts to hydrolyze lipids.

Lipase is the correct answer because it is the enzyme that is responsible for digesting fats in the small intestine. Trypsin, chymotrypsin, and pepsin digest protein. Amylase digests carbohydrates.

Secretin is released from the duodenum in response to an influx of chyme, containing digested food and stomach acid, from the antrum of the stomach. In order not to damage the walls of the small intestine, the pancreatic acinar cells produce and secrete bicarbonate to mediate the pH of the chyme entering the small intestine. Secretin stimulates this release of bicarbonate from the pancreatic cells.

Gastrin stimulates acid secretion in the stomach, trypsin breaks down amino acids, and cholecystokinin increases contraction of the gallbladder to expel bile salts into the lumen of the duodenum through the sphincter of Oddi.

Pepsin and gastrin are secreted into the acidic environment of the stomach, while lipase, trypsin, and carboxypeptidase are secreted into the basic environment of the small intestine. Acidic chyme from the stomach must be neutralized in the duodenum before the enzymes in the small intestine can begin digestion, creating the basic environment for these enzymes to function.

Lipase does not function in an acidic environment.