How to Edit Question Choices in Master Slide

The "Question choice" box accepts input based on the type of question slide that is inserted. The separate response buttons will appear within the "Question choice" box.

You can adjust the width of the "Question choice" box on the Slide Master Question layout. Then, when you insert new question slides, the responses will be at the adjusted width.

Existing question slides don't automatically switch to the adjusted layout. But you can switch each one quickly by reapplying the layout from the "Apply Layout" button on the HOME ribbon.

Note: In a new Storyline file, the default Slide Master includes 2 question layouts: one with just text placeholders, and one that also includes a media placeholder. If some of your questions have pictures and some don't, set up both types of Question layouts, and then apply the layouts as needed. 

https://www.articulatesupport.com/article/Storyline-360-Using-Slide-Masters