The best way to turn in one part of google slides, since you can't split the slide when turning assignments on classroom, is to copy and paste the part you want to turn in into another slide and submit that portion. I've done for many projects before and if you name the different slides (Ex: Assignment Portion One 5/6/20) so it won't get confusing with the multiple slides.
This is the best way I can think of, I hope I helped, and please correct me if there is a better way!
