Answer choice B?

B doesn't completely fix the flaw here. The author is trying to prove that they are non native, and saying that many were probably brought by humans might slightly strengthen the argument, but it doesn't go far enough to prove it correct. We need to establish that they are all non native, and to do so, we need to establish what it means to be native or not. By the authors reasoning, anything that traveled at any point must not be native. This doesn't have to be true, by that reasoning all land animals are nonnative since we originally came from the sea. We need an argument that establishes that native means what the author thinks it means.

Last sentence should be answer choice, not argument