The correct answer is option D. "It activates lymphocytes".
Explanation:
Vaccines based on weakened pathogens work by activating lymphocytes without producing a major infection, in what is known as producing an immunostimulating effect. This is the case for the flu shots, the weakened viruses activate lymphocytes to produce antibodies while the viruses are quickly eliminated. Next time the person is in contact with the flu virus, the immune system will be protected with the memory cells that were produced by activating the lymphocytes.
