Which sentence from Tim O'Brien's chapter "Good Form" best shows that his work is an example of metafiction?

I found the choices for your question and the correct answer would be "But listen. Even that story is made up."

Metafiction basically means that the work itself is referencing its own construction and the reader is aware and knows, and the book keeps reminding him of it, that the story is a piece of fiction. What it means is that the author is breaking the fourth wall and talking directly to the reader. That is why this answer is correct.