Final answer:
A hapten is C. A small chemical compound which reacts with preformed antibodies but is not immunogen on its own. It must bind to a larger carrier molecule to become immunogenic and provoke an immune response.
Step-by-step explanation:
In the context of immunology, a hapten is a small chemical compound that can bind to antibodies but is not immunogenic by itself. This means a hapten cannot provoke an immune response unless it is attached to a larger carrier molecule, often a protein. Once bound to a carrier, the complex can act as a full antigen, and the immune system may produce hapten-specific antibodies that recognize and bind to the hapten, even when it's not attached to the carrier molecule.
There are various molecules which can be identified as haptens, leading to allergic reactions, such as urushiol from poison ivy or penicillin which can cause drug allergies. Therefore, the correct answer to the student's question is C. A small chemical compound which reacts with preformed antibodies but is not immunogen on its own.