Which of the following medications is considered an antacid?

Calcium carbonate is recognized as an antacid because it neutralizes stomach acid, providing relief from heartburn, indigestion, and upset stomach. It works by directly reacting with the hydrochloric acid in the stomach to reduce acidity. This makes it effective for acute symptomatic relief of acid-related gastrointestinal issues.

Other medications listed serve different purposes. Omeprazole is a proton pump inhibitor, which reduces the production of stomach acid but does not neutralize existing acid directly. Diphenhydramine is an antihistamine primarily used for allergies and as a sleep aid, and Metformin is an oral medication for managing blood sugar levels in type 2 diabetes. Thus, these options do not fit the definition of an antacid.