Which of the following medications is classified as an H2 blocker?

Famotidine is classified as an H2 blocker, which stands for histamine H2 receptor antagonist. These medications work by blocking H2 receptors in the stomach lining, which are responsible for stimulating the production of gastric acid. By inhibiting these receptors, famotidine effectively reduces stomach acid secretion, making it useful in the treatment of conditions like gastroesophageal reflux disease (GERD), peptic ulcers, and Zollinger-Ellison syndrome.

In contrast, the other medications listed serve different purposes. Fexofenadine and levocetirizine are antihistamines primarily used for allergic symptoms; they target H1 receptors rather than H2 receptors. Diphenhydramine, also an antihistamine (H1 blocker), is commonly used for allergies and as a sedative. This distinction in the mechanism of action and receptor specificity highlights why famotidine is the correct choice as an H2 blocker.