Is there a reason that " cosa tu mangi" would be incorrect? i understand that it is unnecessary because 'mangi' has an implied 'tu' in it, but is it INCORRECT to put the 'tu' in there? Or would it be technically CORRECT, just UNNECESSARY?
"Che cosa mangi" is what I use colloquially. Why wasn't that offered as an option? Seems a Duolingo teacher/editor needs to jump in and fix/add to this. Why leave students to wonder? I don't see any in this forum.
No, it's not correct: to emphasise the subject in a what/which question you either put it at the beginning (tu cosa mangi?) or at the end for a stronger emphasis, i.e. you as oppose to others (cosa mangi tu?).