Answer:
Those most susceptible to infectious diseases and for whom vaccination is available are very young or very old people.
Step-by-step explanation:
Infectious diseases tend to occur more frequently and relatively severely in the extreme age groups, i.e., in childhood and old age.
Although everyone is susceptible to acquiring an infectious disease, vaccines will always be available for very young and very old people, as they are considered to be at higher risk of infection.
Vaccines are preparations that contain fractions of microbes, without the capacity to produce disease, to stimulate the immune system, and to be able to fight the infections they produce.
With regard to the other options, which are not correct:
a. Anyone who is not vaccinated : anyone can be vaccinated, especially if they have not been vaccinated, but the most susceptible are given priority.
c and d. People in developing or developed countries must receive the vaccines established by their health system, always guaranteeing the immunization of the most susceptible.