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.

Alpha-amylase is a component of saliva and is responsible for the initial digestion of carbohydrate and starch molecules. Chewing food increases the surface area over which alpha-amylase can act on ingested starches, preparing them for further digestion by the stomach.

Trypsin and pepsin break down proteins, while lipase breaks down fats. Pepsin becomes active in the stomach, while trypsin and lipase are mostly active in the small intestine.

The correct answer is carbohydrates.

Salivary amylase can only digest carbohydrates. Proteases further along in the digestive pathway breakdown proteins, while lipases digest fats. Amino acids are the product of digested proteins.

Salivary amylase is produced by the salivary glands in the mouth. Its primary role is to break carbohydrates into simple sugars so that they can be digested further after passing through the oral cavity.

Lingual lipase is responsible for the digestion of lipids (fats), not carbohydrates. Carbonic anhydrase proteins are responsible for reacting gaseous carbon dioxide to form bicarbonate or carbonic acid. Salivary carbonic anhydrase removes the gas from ingested carbonated drinks; it does not participate in the digestion of food. Mucin is a protein found in saliva and mucous linings, and is designed to add viscosity to these secretions. It serves no enzymatic function.

The stomach houses a number of highly specialized cells that aid in digestion. G cells secrete gastrin, which stimulates parietal cells to secrete hydrochloric acid. Chief cells secrete pepsinogen, which interacts with the acid to generate active pepsin. Pepsin then acts on ingested proteins and begins to cleave them.

Goblet cells secrete the mucous lining of the stomach.

Protein digestion begins in the stomach, where it is exposed to the protein pepsin. Pepsin only serves preliminary digestive processes. Protein fragments are further broken down by proteases in the small intestine, allowing amino acids and small protein fragments to be absorbed into the bloodstream.

Carbohydrate digestion begins in the mouth with salivary amylase. Lipid digestion and nucleic acid digestion begin in the small intestine.