Change line spacing in pop-up caption in master slide of MCQ question slide

Have changed one word in the title: there is no master view, you are talking about the quizzing master slide.

My first question is: are you using captions or shapes for the feedback messages? If you want to work from the master slide, not from the Object Style Manager, you can see the used style when selecting a message in the master slide (beware: they are all stacked in the same location, both for responsive fluid boxes and non-responsive projects). You will probably see that they are using exactly the same object style as all feedback messages for interactive projects. Edit the line spacing, which will make the + sign appear before the style name, indicating that you did override the default style. Use the Hamburger (or hotdog) menu to save changes to existing style.

There is an old bug, which I log for each version. The better way would have been to use Quiz Preferences, Defaults but that is limited to caption styles, you cannot choose shape styles.

You can also perfectly use the Object Style manager, once you have identified the default styles. I always do that in this case, because want to have line spacing for all text containers to be set at least to 1.2 or 1.3.