Which of the following medications is known to act as an antacid?

Calcium carbonate is well-known for its use as an antacid. It works by neutralizing stomach acid, thereby providing relief from indigestion and heartburn. This medication increases the pH level in the stomach, reducing acidity and alleviating associated symptoms. It can also serve as a calcium supplement, addressing deficiencies while providing its antacid properties.

Other options such as warfarin, lidocaine, and diphenhydramine do not have antacid properties. Warfarin is an anticoagulant used to prevent blood clots, lidocaine is primarily a local anesthetic, and diphenhydramine is an antihistamine used for allergy relief and to prevent motion sickness. Each of these medications serves different therapeutic purposes, but none of them function to neutralize stomach acidity like calcium carbonate does.