Duo's goal is for you to be able to translate documents on the web. To get you to that point they have to use simple sentences and fragments of speech that reinforce learned material. Consequently some of questions presented will seem awkward when translated.
La nourriture est mauvaise when translated to the food is bad sounds to the English speaker's ear as if it was referring to a known example of food, often something actually present.If it is present he would probably turn his gaze to the food to look for obvious signs of badness.
Nourishment and Diet, on the other hand are not likely to cause an English speaker to look at the food to see if he can spot the deficiency in the recommended daily amount of vitamin C or whatever. He is more likely to speculate about the process whereby food is selected or prepared.
The average French speaker overhearing some patrons of a restaurant complain that La nourriture est mauvaise would not think it's ok for him to eat there because he isn't worried about getting nutritional deficiency diseases like rickets etc.