Why must your research question be open-ended?

Because when researching something you are supposed to take notes and soak up knowledge, a simple multiple choice question does not prove that your are familiar with the topic you are being tested on.
Hope this helps!

Open-ended questions will give you higher marks!
Just kidding. Open-ended questions can make people think, and have room for imagination. If research question are all "yes-or-no" questions, your research won't be as successful since you can't get a detailed answer. Hope this helps :)

The answer would be d.hope this !