C. Attach the fragment to sentence (2).
One of the common causes writers have for the error of the sentence fragment is because many people write like they speak. And, when speaking, people tend to add ideas after a main thought is spoken. Fragments will sometimes occur because the content is a sort of afterthought for the idea that precedes it. When looking to correct fragments, check to see if it appears to be an extension of the sentence preceding it, and if so, it can be corrected by simply attaching it to the preceding sentence, which is the case here.