The correct answer is B. Antigen
Explanation:
An antigen refers to any strange or harmful substance such as a virus, bacteria or pollen that leads to a response by the immune system. This implies once an antigen enters the body or it is identified in the immune system white cells or lymphocytes attack by releasing antibodies.
Additionally, antigens are usually depicted as small circles or rods move freely or are attached to antibodies that have a "Y" shape because once antigens enter the body these are bound by antibodies. This implies, in the case presented the red figure indicated by the arrow is the antigen, while the grey figure that attaches to it is the antibody reacting to the antigen. This can be explained as antibodies with the "Y" shape only bind with antigens either to help to its recognizing or destruction.
