Final answer:
Hydrochloric acid (HCl) is the stomach secretion responsible for activating pepsin, destroying bacteria, and breaking down connective tissue in food.
Step-by-step explanation:
The stomach secretion that activates pepsin, destroys bacteria, and breaks down connective tissue ingested is HCl (Hydrochloric acid). Parietal cells in the stomach lining secrete hydrochloric acid, which creates a highly acidic environment with a pH typically around 1.5 to 2.5. This acidity is crucial because it denatures food proteins, facilitating their breakdown, and activates pepsin from its inactive form, pepsinogen, secreted by chief cells. Besides, the acidic environment created by HCl serves as a defense mechanism against ingested microorganisms, thereby reducing the risk of infection. Additionally, HCl helps convert the inactive enzyme pepsinogen into its active form, pepsin, which catalyzes the hydrolysis of peptide linkages within protein molecules.