Antigen is a substance which

The correct option is C because an antigen is a molecule or substance, such as a protein on the surface of a virus or bacteria, that the body recognizes as foreign. When an antigen enters the body, it triggers the immune system to produce antibodies to fight off the perceived threat. While antibodies destroy bacteria or neutralize toxins, the antigen itself is the trigger that initiates this defensive response. Therefore, its primary function in the context of the immune system is to act as a stimulus for an immune reaction.