A vaccine can protect you against a disease because it
(1)destroys toxic substances from bacteria before they can make you sick\
(2) stimulates your immune system against the pathogens
(3) kills any pathogenic bacteria in your body
(4) changes pathogenic bacteria into harmless bacteria
IS THE ANSWER (4)
I don't agree.
http://www.cdc.gov/vaccines/vac-gen/howvpd.htm
No, the answer is not (4) "changes pathogenic bacteria into harmless bacteria."
The correct answer is (2) "stimulates your immune system against the pathogens."
When you receive a vaccine, it contains small amounts of weakened or inactivated pathogens (such as viruses or bacteria) or pieces of them called antigens. These antigens are recognized by your immune system as foreign invaders.
When your immune system detects these antigens, it mounts a response by producing specialized cells and molecules that fight off the pathogens. It creates antibodies that specifically target and neutralize the pathogens. It also stimulates the production of memory cells, which "remember" the pathogen and enhance the immune response if you are infected in the future.
By stimulating your immune system in this way, vaccines teach your body how to recognize and effectively fight off the actual pathogen if you are exposed to it later. This helps prevent you from getting sick or reduces the severity of the illness.
Option (1), "destroys toxic substances from bacteria before they can make you sick," is not the primary mechanism of a vaccine. Vaccines work by training your immune system, rather than neutralizing toxins directly.
Option (3), "kills any pathogenic bacteria in your body," is also not the primary purpose of a vaccine. Some vaccines do contain inactivated or weakened bacteria, but their main goal is to stimulate your immune system, rather than directly killing the pathogens.
In summary, vaccines primarily work by stimulating your immune system to recognize and fight off pathogens, protecting you from getting sick or reducing the severity of the illness.