A client frequently takes diphenhydramine (Benadryl) to alleviate his upper respiratory symptoms associated with allergies. Which of the following mechanisms of action applies to diphenhydramine (Benadryl)?

A. Diphenhydramine is an H1 antihistamine that works by blocking the effects of histamine at H1 receptor sites, thus alleviating allergy symptoms. B. H2 receptor sites are primarily involved in gastric acid secretion, and diphenhydramine does not affect these receptors. C. Diphenhydramine does not stimulate histamine effects; it blocks them instead. D. The medication does not occupy all H receptor sites; it specifically targets H1 receptors to exert its effects.