What is a substance used to stimulate an animal to build up resistance to a disease called?

The correct answer is vaccine. A vaccine is a biological preparation that provides active acquired immunity to a particular infectious disease. It usually contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the germs, their toxins, or one of their surface proteins. When administered, vaccines stimulate the immune system to recognize the agent as a threat, destroy it, and remember it for future defense. This process helps the body build up resistance to the disease, preparing the immune system to fight off infections more effectively if it encounters the actual pathogen in the future.

The other choices, while related to disease treatment and prevention, do not specifically address the process of stimulating immunity. Antibiotics are used primarily to treat bacterial infections rather than to prevent them through immune response. Inoculation refers to the practice of introducing a vaccine into the body but is a more general term and does not define the substance itself. Serum, such as antiserum, contains antibodies that are used to fight infections but does not prime the immune system for future response in the same way a vaccine does.