"Soup is tasty food."
Translation:Zupa to smaczne jedzenie.
That doesn't mean that in will work in every grammar construction, or even every sentence where it's shown in the hints. The hints are for the whole course.
Nevertheless, here you can answer with "Zupa to smaczne jedzenie" or "Zupa jest smacznym jedzeniem". See here for more info: https://forum.duolingo.com/comment/16373167