Animals pass down their features to their children when they breed. When you separate a population (or group of animals of the same species), say by building a large wall, they can't interbreed with each other. This means that if an animal on one side of the border gets an adaptation, it won't pass it on to the other animal. Soon, each side will evolve different adaptations, and the two populations will become so different that they become two different species.
It is possible that one-half wouldn't be able to survive, and go extinct, but it doesn't happen all the time (A is incorrect). Similarly, the populations don't ALWAYS evolve into two different species, though they may (B is incorrect). However, if the environments differ enough, they will acquire different adaptations through evolution. C is correct! Keep in mind that D is incorrect: the first part, the populations can't interbreed, is correct, but that doesn't mean they will die off.
