Vaccines have the goal to teach our immune system to identify disease-causing microorganisms and thus have a fast response against them, preventing us from infection.
This is made by presenting proteins from the microbe, known as antigens, which are then recognized by the immune system and trigger the production of an appropriate antibody to fight the infection.
Vaccines can be made of killed microbes, fragments of the microbe, or weakened microbes.
This means the sentence would be completed as follows:
Inactivated vaccines work by introducing certain molecules from the pathogen known as antigens so that the immune system can safely leam to recognize them as hostile invaders and produce antibodies to fight them.
And the right answer would be b, antigens; antibodies.