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

A sign of an allergic reaction is represented by the redness and swelling of the skin. This response is a typical manifestation of the immune system reacting to an allergen, which is a substance that the body mistakenly identifies as harmful. When an allergen is encountered, the body releases histamines and other chemicals, leading to inflammation and blood vessel dilation. This process results in the characteristic redness and swelling, often accompanied by itching or hives.

In contrast, increased energy levels, clear skin with no symptoms, and increased appetite do not indicate an allergic reaction. Increased energy levels can be associated with various factors, including well-being or stress relief, and do not correlate with skin reactions. Clear skin with no symptoms suggests that there is no allergic reaction occurring as there are no visible signs of inflammation or irritation. Increased appetite can be influenced by numerous conditions or lifestyle changes, but it is not a sign of an allergy. Therefore, the correct indication of an allergic reaction is the presence of redness and swelling on the skin.