"You are very beautiful."
Translation:तुम बहुत सुंदर हो।
Maybe because तु is pretty disrespectful, unless you are talking to a child. So it would be a little strange to disrespectfully say "You are very beautiful". Just a guess.
Doubt it because the course already uses तू disproportionately and this seems like one of the few instances where the intimate form might actually be used (when you are talking with your SO or a close family member for instance).
I'm guessing it was marked wrong because Steve used तु instead of तू.