Hi there,
We all know that Subjunctive mood express various states of unreality such as wish, emotion, possibility, judgement, opinion, obligation, or action.
The answer to this is: I demand that they be punctual.
The reason that is the answer is because as you can see that sentence has a lot of emotion and action because you are demanding and demanding can be a form of a subjunctive mood.
Hope this helped :)
Have a great day