Which of the following is a sign of an allergic reaction?

A sign of an allergic reaction includes the presence of hives, which are raised, itchy welts on the skin that can appear in response to allergens. Hives, also known as urticaria, arise due to the release of histamine and other chemicals from mast cells in the skin, often as part of the body's immune response to a perceived harmful substance.

While dry skin, headache, and fatigue can be symptoms associated with various conditions, they are not specific indicators of an allergic reaction. Dry skin is typically related to environmental factors or skin conditions, a headache may stem from numerous non-allergic causes such as stress or dehydration, and fatigue can result from multiple underlying health issues or lifestyle factors. Therefore, the presence of hives clearly identifies an allergic response, making it the correct answer.