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.

Pepsin is a protease that is secreted by the chief cells located in the stomach. All of the other choices are digestive enzymes secreted by the pancreas. Lipase helps to digest fats, carboxypeptidase and chymotrypsin both break down proteins, and trypsinogen is a zymogen of trypsin.

Trypsin is created by the pancreas, and is responsible for the degradation of proteins in the lumen of the small intestine. More importantly, activated trypsin activates all of the other enzymes present in the small intestine. If trypsin is absent, virtually no digestion would take place.

The pancreas is responsible for secreting both lipase and amylase, which are needed to break down fats and starches, respectively. The pancreas is stimulated to release its respective enzymes about two to three hours after a fatty or carbohydrate-rich meal. These enzymes enter the duodenum through the ampulla of vater. In the duodenum, ingested fats and sugars are broken down to be absorbed later in the jejunum.

Gastrin is released by G cells and stimulates the release of hydrochloric acid in the stomach. Chief cells release pepsinogen into the acidic environment, allowing the conversion of pepsinogen to pepsin.

Lipase is produced by the pancreas in response to the ingestion of meals high in fat, and is responsible for cleaving fats into monoglycerides and triglycerides. Proteins are broken down by pepsin and trypsin, nucleotides are broken down primarily by high acid content, and starches and carbohydrates are broken down by amylase.

All pancreatic enzymes are secreted into the duodenum in zymogen form in order to prevent autodigestion of the small intestine. Trypsinogen is autoactivated to trypsin, which then proceeds to activate the other enzymes released from the pancreas, such as chymotrypsin, amylase, lipase, and the enzymes that break down digested DNA and RNA.