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.

A vaccination works by activating the body's natural defenses.

This happens because even though the live virus isn't actually injected into a person, it's "training" the immune system to building it's defense system against that particular antigen and makes antibodies to help protect you in the future. Some vaccinations create permanent antibodies toward the vaccinations, like varicella and some vaccinations create temporary antibodies like a rabies injections (which is good for 10 years). Some only last a year  because of different strains are so rapidly mutating, like the influenza vaccination.!

I hope this helps!