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.

Cholecystokinin (CCK), made by the I cells of the small intestine and secreted into the duodenum, serves to stimulate contraction of the muscular layer of the gallbladder. CCK is generally released within two to three minutes of ingesting food, especially foods that are high in fats and triglycerides. As the gallbladder contracts, it releases bile salts into the duodenum that help solubilize ingested fat for absorption in the jejunum of the small intestine.

Secretin stimulates release of bicarbonate from the pancreas, gastrin stimulates release of acid in the stomach, and chymotrypsin cleaves peptide bonds of ingested proteins. 

Calcitonin is a hormone secreted by the thyroid in response to increased blood-calcium levels. It acts on the cells of bone to inhibit osteoclast activity (bone break down) and promote osteoblast activity (bone synthesis).

Acetylcholine and substance P are both neurotransmitters involved in orad contraction during peristalsis. Nitric oxide is a neurotransmitter involved in caudad relaxation during peristalsis.

The answer to this question is cholecystokinin (CCK). CCK is responsible for the release of bile from the gallbladder. The bile is responsible for fat breakdown and absorption in the small intestine, as the bile acts to emulsify fats so that lipase can effectively digest them.

Gastrin and secretin are also digestive hormones, but serve different functions. Gastrin promotes acid release from parietal cells in the stomach, while secretin suppresses acid release. Renin is not involved in digestion, and is released in response to low blood pressure.

Bile is released by the gallbladder due to secretion of cholecystokinin (CCK) in response to lipid digestion. Cholecystokinin is produced by I cells of the small intestine and has a number of digestive regulation effects, including pancreatic and liver stimulation. The liver produces bile, which is stored in the gall bladder until stimulation with CCK cause gall bladder contraction and bile release.

The vagus nerve indirectly stimulates the parietal cells by releasing the hormone gastric releasing peptide (GRP). GRP then acts on the G cells, stimulating them to secrete gastrin. Gastrin then acts on the parietal cells to promote hydrochloric acid secretion. Since there is no stomach, this cascade cannot proceed, and thus no acid would be secreted.