Question

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.

Answer 1

That has an undetermined answer.

A good research question addresses a gap in knowledge (not something already thoroughly covered), is answerable (not impossible), and is feasible but not trivial (not something easily known). It should be clear, focused, significant, and researchable.