Answer: Freewriting.
The other answerer is wrong. Outlining isn't brainstorming, it's just organizing.
I included a screenshot from my schoolwork that proves that this answer is correct.
More proof, an excerpt from my school lesson about compare and contrast topic ideas (i underlined the important parts):
If you are not sure what to write about, or if you have not been given a specific topic, there are a couple of ways to generate topic ideas: freewriting and listing.
Freewrite for a few minutes. When freewriting, you should write without stopping to think or edit—just write about whatever comes to mind.
