The answer is A. Decrease in genetic variation
A founder effect is a result of a geographical separation of a few individuals from the original population. Those few founding individuals will form a new population so they are founders. In this new population, there will be present alleles of the few founding individuals. However, those few founding individuals do not represent a whole genetic variation of the old population, but only some part of it.
Imagine a population of fish in which there are yellow, green, blue, and red colored fish. In the population, there is a genetic variation. But, if a few green-colored and red-colored fish geographically separate from the population and form a new population, in the new population there will be decreased genetic variation (because there are only green and red fish compared to the old population where are fish in 4 different colors).
