Bile is released from the gall bladder into the small intestine, where it helps with the emulsification of fats. Bile salts and phospholipids are crucial amphipathic compounds that allow lipids to associate while in the aqueous environment of the small intestine. Cholesterol promotes fluidity and prevents the lipids from becoming tightly packed. Bilirubin is a byproduct produced in the liver and secreted in bile; it is later reabsorbed in the ileum and returned to the liver in a cyclic pattern.

Proteases are secreted from the pancreas and assist in protein cleavage in the small intestine. They are not involved in bile or lipid digestion.

The correct answer is parietal cells. Parietal cells are responsible for HCl secretion in the stomach, which lowers the overall pH of the stomach. Chief cells secrete pepsinogen, which is converted to pepsin and is responsible for digesting proteins. G cells secrete gastrin, which stimulates parietal cells.

Only one of the enzymes listed functions in the stomach, where high acidity results in a low pH. That enzyme is pepsin. Other enzymes listed function in the small intestine, and will be denatured by high acidity.

Digestion of proteins begins in the stomach. The low pH of the stomach converts pepsinogen into pepsin, which is then used to break down proteins.

In the stomach, food is broken down using a host of different cells and enzymatic processes. Chief cells release pepsinogen, making it the correct answer. Parietel cells, which release HCl, allow the conversion of pepsinogen (a zymogen) into pepsin. Pepsin actively digests proteins in the highly acidic environment of the stomach.

Goblet cells release mucus which lines the stomach for protection.

In order to stimulate digestion in the stomach G cells secrete gastrin, which stimulates parietal cells. Pepsinogen is released by chief cells. The parietal cells release hydrochloric acid into the stomach lumen. The lowered acidity cleaves pepsinogen and creates pepsin, which begins to degrade proteins. 

Mucous cells have the nondigestive role of lubricating the stomach lumen and protecting the stomach epithelium from degradation by the highly acidic gastric juices.

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.