Solution:
According to the problem, we have that rabbits produce large numbers of offspring during each reproductive season, yet the number of rabbits within a given population changes very little from year to year.
This can be explained because the stability of the population size is most likely the result of environmental factors that keep the population in check. Rabbits are seen as food by many predators which will keep the numbers down. The principle of the survival of the fittest will ensure that young strong rabbits get to breed and carry the gene pool to the next generation.
So that, we can conclude that the correct answer is:
environmental factors that keep the population in check.