Final answer:
B) Exogenous antigen. Exogenous antigen is useful for helping the immune system recognize an extracellular pathogen. It is taken up by antigen-presenting cells, processed, and presented to the immune system, triggering an immune response.
Step-by-step explanation:
The correct answer is B) Exogenous antigen. Exogenous antigens are antigens that are derived from outside of the body, such as from extracellular pathogens. These antigens are taken up by antigen-presenting cells, processed, and presented to the immune system, triggering an immune response. The immune system recognizes exogenous antigens as foreign and mounts a response to eliminate the pathogen carrying these antigens.