The correct answer here is D. All of the phrases given here are semantic markers that can be used to indicate that a summary is about to be presented.
C is obviously correct. Any time that a person says or writes “to summarize…” it is clear that a summary is coming. But the others are correct too. When you say “in other words,” we can tell that you are about to re-present an argument that has already been made. This is a form of a summary. When you say “It amounts to this…” you are also about to recapitulate the main points that you have already made. Therefore, D is the best answer.