Which of the following can be a symptom of a product allergy?

The symptoms of a product allergy often include redness, itching, or a rash after the application of the product. These symptoms indicate that the body's immune system has reacted adversely to an ingredient in the product, leading to localized inflammation. This reaction can occur shortly after exposure and typically manifests as visible irritation of the skin, which aligns with the types of responses that characterize allergic reactions.

In contrast, hair falling out immediately could point more towards breakage or damage rather than an allergic reaction, as hair typically does not shed immediately after application of a product. Excessive shine is a typical desirable outcome from certain products and not a symptom of an allergy. Lastly, split ends are also a sign of hair damage rather than an allergic response and do not indicate a skin-related reaction. Thus, the first choice accurately represents a symptom indicative of a product allergy.