Definitely nel/nello/nella; there's even a verb impiattare (in + piatto), though it's a neologism and mostly used among cooking savvy. "Sul piatto" means on top of the plate, and although never really wrong, the only context that comes to mind is when the waiter carries it on a plate.
The one thing that I know for sure is that whichever word DL uses, some of the comments will tell them they are wrong.
That's part of why I like the comments. We have a great opportunity to learn. We have some very knowledgeable people who take their time to explain things to us.
I just want to thank everyone for the constant questions and answers.
In previous sections including Food and Food 2 it has always been 'nel piatto' instead of 'sul piatto' and native Italians have backed 'nel piatto' as being the correct way to say 'on the plate'. In this section of level 4 Prepositions this is the third time I have seen 'sul piatto so have reported it.