Which of the following is an example of a histamine receptor antagonist?

Ranitidine is classified as a histamine receptor antagonist, specifically targeting the H2 receptor. This mechanism of action helps to reduce gastric acid secretion in the stomach by blocking histamine's effect on these receptors. As a result, ranitidine is commonly used to treat conditions such as gastroesophageal reflux disease (GERD) and peptic ulcers, where decreased gastric acidity is beneficial for healing and symptom relief.

In contrast, omeprazole is a proton pump inhibitor that works by blocking the final step of gastric acid production rather than antagonizing histamine receptors. Calcium carbonate and magnesium hydroxide are antacids; they neutralize stomach acid but do not affect histamine receptors or their signaling pathways. Thus, ranitidine is the most appropriate example of a histamine receptor antagonist among the choices.