That looks like a fragment of a sentence, so the answer should be D. Those words alone are not a sentence, so they can't be B or C and since it's not a sentence and is short, it can't be a run-on either.
Answer:
If you want to impress your teachers, then you would want to write a couple of paragraphs. If not, then you would only want to write a sentence.
Explanation:
In the sentence the tone seems to be excitement.
