Final answer:
Option C is a run-on sentence because it lacks proper punctuation or conjunction between two independent clauses.
Step-by-step explanation:
The correct answer to the student's question is option C. "The barn has a funny smell maybe it's coming from the cows." This is a run-on sentence because it consists of two independent clauses incorrectly joined without proper punctuation or conjunction. To correct it, you could insert a period or a semi-colon between the clauses, add a coordinating conjunction, or subordinating clause.