Hello! Here's how to get rid of the gradient:
1- Open the question and click Slide View.
2 - Click the Design tab at the top.
3 - Click the Background Styles dropdown and choose Format Background.
4 - For the Fill option, choose Solid Fill. (And you can also choose a fill color here, or just leave it as white.)
5 - If you only want to remove the gradient on that one slide, you can just click Close. If you want to remove it from all slides, first click Apply To All, and then Close.
Hope that helps!
