Answer: A. a molecule that reacts specifically with an antibody or immune lymphocyte.
Step-by-step explanation:
An antigen is a foreign body that enters inside the other host body it initiates an immune response. As a result of action of immune response the antibodies are produced. The antibodies are the proteins which fights against the antigen. The antibodies are produced by the white blood cells or lymphocytes. The antigen can be in the form of chemical agents, pollen, pathogens such as bacteria and viruses.