Which substance is produced in the parietal cells of the stomach?

The substance produced in the parietal cells of the stomach is hydrochloric acid (HCl). Parietal cells, found in the gastric glands lining the stomach, are specifically responsible for the secretion of this strong acid, which plays a critical role in digestion. HCl helps maintain an acidic environment in the stomach, necessary for activating digestive enzymes, such as pepsin, and aiding in the breakdown of food. The acidic conditions also help kill harmful bacteria that may be ingested with food.

In contrast to HCl, digestive enzymes are predominantly produced by chief cells in the stomach, while bile is produced in the liver and stored in the gallbladder, and saliva is produced by the salivary glands in the mouth. Therefore, HCl is the correct answer, as it specifically highlights the function of the parietal cells in the stomach's digestive process.