Main Idea.
The Main Idea is usually mentioned within the first few lines of the Passage, or whatever you're reading.
That way you know what kind of audience your writing for you can use different words with a lot more complexity. It is completely different writing for kindergarteners and writing a paper for a college seminar.
The best graphic organizer to use to better understand a definition passage is a Venn diagram an idea web a timeline an illustration hierarchy
Answer: Which (A)
It makes more sense with ‘which’ then any of the other options.