Answer:
The correct answer is: the antibodies' shape.
Step-by-step explanation:
Antibodies are proteins called immunoglobulins that serve the immune system by responding to a specific antigen and generating the proper counter to fight it.
Each antibody consists of four polypeptides that form a "Y" shaped molecule. The Y shape is given by two heavy chains and two light chains.
The tips of the Y are composed of 110-130 amino acids that vary greatly among other antibodies. This gives the antibody its specificity.