Answer:
a. Fragment
Step-by-step explanation:
A sentence fragment occurs when a group of words does not express a complete thought because it lacks a subject or a verb, or it is a subordinate clause. Fragments look like sentences but they need at least one independent clause be a sentence.
In this case, the fragments must be joined in one single sentence and separated by a comma:
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 run-on sentence is incorrect because it consists of two or more independent clauses without a word to connect them or a punctuation mark to separate them.