Question

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

Answer 1

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.