The correct answer is immunization or vaccination.
Explanation:
Immunization is the method of developing immunity or resistant to a particular infectious disease, generally by shots or vaccines. Vaccines trigger the immune system of an individual to prevent the person against that infectious disease or pathogen.
The vaccine for yellow fever is the weakened form of the virus provided to the individual in a single shot. It can be given to people travel to yellow fever prone areas such as Africa.
Thus, the correct answer is immunization or vaccination.
