Answer:
I think it's B. survival of the fittest since it's mostly just a phrase people say. I don't think underpopulation because, I'm pretty sure that's used when there not enough of an animal in an area, so while it could be the answer I don't think it is. And overpopulation is used when there's to much of an animal in an area so like with underpopulation it could be the answer but I do not think it is. And Variation is a step in Natural Selection so that's definitly not it. So since Survival of the fittest is mostly just a phrase people say I think that's the answer. Sorry for wasting your time if I'm wrong.