Translation:a cat and the cat
I think in this exercise, Duolingo is trying to show you the difference and comparison between definite and indefinite nouns in Arabic and English.
In English, typically, an indefinite noun comes after "a/an" - Rarely though in some general sense the word is mentioned without "a/an", like for example I love nature (the word nature here comes without any indefinite article because it is a general word for a general thing that cannot be counted).
Apart from giving us (non-learners or advanced learners) a chance to boast our knowledge, this sentence is useless for actual Arabic learners. What usually works in interactive teaching is parallel structures about which a student can reflect and infer the grammar by herself. For instance قطة كبيرة a big cat القطة كبيرة the cat is big and القطة الكبيرة the big cat. Mixing grammatical structures artificially in one nonsensical sentence, which seems to be a basic principle in this Arabic course, is not good practice (unless your goal is to set up a debating arena for experts). It only creates confusion: learners do not have the necessary elements to do the analysis.