Lake superior is one of the world’s largest bodies of fresh water. with part of it in the united states and part in canada. a. fragment b. run-on c. correct

The correct answer is A. fragment.
Although the first sentence is a correct sentence (it is complete, has a subject and a verb), the second one is not - the second part is not even a sentence given that there are no subjects or verbs in it, which means it is only a fragment, unable to exist on its own, outside of a larger sentence. 
B is incorrect because that would mean both of these sentences are in one large sentence, but not connected properly. And obviously, C is also incorrect.