Question

How does a vaccine work? a. It activates the body’s natural defenses. b. It weakens the cell walls of bacteria, causing the cells to burst. c. It treats the symptoms of an infection. d. It attacks antibiotic-resistant bacteria.

Answer 1

It seems to be a. Activates the body's natural defenses.
Here is an explanation about this:
A Vaccine is a dead or severely weakened virus that allows your body to create antibodies (which destroy the viruses), so when you get the actual virus, your body is ready.

Answer 2

I would say that it activates the body's natural defenses