Which primarily provides thoughts or opinions about the topic? the transition word
the refining sentence
the sentence that illustrates
the concluding sentence

It would not be concluding, because that would be the end of a topic, or a prompt etc. so you can rule that out, a transition word is a word that can help make a sentence more powerful, so vote that out, and now you are left with the refining sentence, and the sentence that illustrates, when you see something that is illustrated, it connects to the story and shows you a visual, so the sentence that shows you a clear opinion, or topic, would be correct, there for IT IS C.