Each of these practices has greatly reduced the spread of infectious diseases in the developed world except
a) vaccination.
b) antibiotics.
c) reduced population growth.
d) water purification.

C) Reduced population growth

Explanation:

Vaccination avoids that an infectious agent causes a disease in a person, so it stops the spread of these diseases. Also, antibiotics kill bacteria responsible for infections in an ill person, therefore, they also end the spread. Water purification eliminates pathogens from the water such as bacteria or fungus, so it helps to prevent infections in a population. Finally, reducing the growth of the population doesn't directly affect the spread of infectious diseases.