It should be what the text is mostly about. The authors purpose is the main idea of the text, or just the idea of what the author is writing. So what the text is mostly about is the authors purpose.
Well, this is a tricky question for sure so I believe it is: What is this text mostly about? I believe this because you must read the passage first to see what it is talking about to find the purpose of this text and possibly the central idea.
