The correct option is D That's the down side
Out of the four options, option B) that's the downside is the only statement that expresses the subject's unhappiness. The following sentence is about the subject not being able to leave until midnight, and that's our hint. After reading that sentence, things get easier as we realize that we need a statement expressing unhappiness. The other three options are positive phrases. So, the correct option is B) That's the downside.