When formulating a research question, a researcher should write a question that has already been thoroughly covered. that is impossible to answer. that can be answered easily. that has an undetermined answer.

I would say the correct answer is that has an undetermined answer.
If a subject has been thoroughly covered, then there is no need for this researcher to do it all over again. If it's impossible to answer, there is no need to try. If it can be answered easily, then it doesn't really require a research. This leaves us with the last option, because a researcher might want to try and determine the answer.