The best answer for that gapfill is 'could', but you can also use 'might'. I think 'could' is the best answer because 'could' expresses possibility, so it is the most natural sounding in this situation.
Personally, I think the gap fill exercise is poorly written though.
I took your question and turned it into a blog article, anyone that wants more information please read about conditionals with subjunctive here.
Student or Learner