How does creating an idea inventory you start a research paper?
it you figure out what you already know and what you need to research about your topic.
it provides an extensive list of possible approaches to take on various topics.
it organizes your ideas for topics into manageable pieces so you can narrow down your topic.

I am pretty sure that the correct answer on the question which is being asked is the first option from the scale. It helps you figure out what you already know and what you need to research about your topic. The word 'idea' in question helps you to understand that you don't need to take extensive list of approaches or organizer of ideas as a basis.