One of the options here was "Ich są jedzenie dla zwierząt". Which, to my understanding (without worrying about cases and such), translates to they are food for animals. So... we did it boys. We figured out where all the missing zoo guests were going.
Also, I don't think it was a bug. Duolingo gave me three Polish sentences and asked me which one meant "this is food for animals." So it's expected that Duolingo throw me some grammatically incorrect sentences.