Honestly, the best answer is diplomacy. As far as the Social Studies curriculum is concerned, the neither the U.S. or Mexico are sending troops to either country so it is not interventionism. Neither country is trying to take over the other so it is not interventionism. Internationalism is irrelevant. So, the best way to describe the relationship is through talking channels the U.S. has with Mexico.
The right answer is diplomacy. But the best answer in this case is interventionism, although I would disagree with the answer choices.