The answer is D because It offers a concise summary of the main point or claim of the essay, research paper, etc. It is usually expressed in one sentence, and the statement may be reiterated elsewhere.
The first one and the third one both look like good answers. But if I had to choose one, I would go with Piston because its talking about technology. The piston is part of an engine so it would have to do with technology. :)
