Endemic diseases result in a decreasing population, so with that said you can rule out choices B and D.
Among the two left, letter A is the most appropriate answer. Not all endemic diseases have an effect on the number of children a population will have. So if we are speaking about endemic diseases in general, the answer is:
A. Endemic diseases can result in decreasing population numbers: many people die form diseases, and others move away from or avoid moving to places where diseases are rampant.
