The stomach contains several cell types, each with a specific mechanism of action.

Chief cells produce pepsinogen, which is converted to pepsin by hydrochloric acid in the stomach. Parietal cells produce hydrochloric acid, allowing this conversion. Goblet cells produce the mucous lining of the stomach that protects the stomach epithelium from damage from the acidic environment of the gastric lumen.

Amylase is an enzyme found in saliva and the small intestine that breaks starches down into sugars. The correct pairing for amylase would be carbohydrates.

Peptidases break peptide bonds, which are what hold amino acids together in proteins. Different peptidases are found in the stomach (pepsin) and the small intestine (trypsin). The correct pairing for peptidase is proteins.

Lipase is an enzyme that catalyzes the hydrolysis of triglycerides into free fatty acids in the small intestine. The correct pairing for lipase would be fats or lipids.

Pepsinogen is an inactive enzyme (zymogen) that is released by chief cells into the stomach lumen. In order to be activated, the enzyme must be at a very low pH. This is accomplished by hydrochloric acid, which is released by the parietal cells in response to stimulation by gastrin. Parietal cells are, thus, most closely linked to the activation of pepsinogen.

Gastrin is released by G cells. Goblet cells secrete the mucous lining of the stomach.

Pepsinogen is released by chief cells in the stomach, and is the zymogen form of pepsin. Pepsinogen requires an acidic environment in order to be converted to pepsin. Parietal cells release hydrochloric acid into the stomach lumen, thus activating the enzyme.

G cells and goblet cells secrete gastrin and mucous, respectively, into the stomach.

G cells secrete the hormone gastrin. Gastrin further stimulates stomach acid secretions via parietal (oxyntic) cells. Chief cells secrete pepsinogen, the inactive form of the enzyme pepsin, which degrades proteins. Goblet cells secrete mucous to protect the cells of the stomach from the acid.

The only one of the enzymes listed that becomes activated in the presence of an acidic (low pH) environment is pepsinogen. When activated, pepsinogen forms into pepsin which occurs in the stomach because of the  (hydrochloric acid) present. Enzymes and/or hormones that have the suffix -ogen, or the prefix pro- are known as zymogens or prohormones, which are not active. Different environments/molecules activate these enzymes and proteins. This method of activation allows for more rapid usage, rather than having to synthesize these molecules from scratch when they are needed.

To become active, hydrochloric acid in the stomach reacts with pepsinogen to make pepsin.

The chief cells of the stomach secrete the proteolytic zymogen pepsinogen. This zymogen then becomes activated as pepsin in the stomach.

The parietal cells of the stomach begin secreting hydrochloric acid even upon the smell of food (before consumption takes place) in order to begin the digestion process. These cells are common targets of antacid medications to reduce acid reflux. 

Hydrochloric acid, produced by the parietal cells of the stomach, activates the zymogen pepsinogen.