The correct option is B Allergy
Antihistamines are drugs that inhibit the action of histamine in the body by blocking the receptors of histamine. When the histamine recepotrs are stimulated by histamine, it may produce allergic reactions, such as itching, hay fever and rash or hives. Antihistamines treat the symptoms of such allergic reactions.
Thus, the correct answer is option (D), 'Allergy'.