You are right, the lesson is about the definite article. This book says: ”DO NOT use "the" in front of plural nouns or when a general statement is made” I understand from here that ”the” is well-used only when we talk about a specific group of cats. The exercise with tiles is presented only for the best answer: ”Cats do not eat vegetables.”
So, does this translate to mean cats in general, or some specific cats that we are aware of and talking about? Does the Romanian use the plural with article like this to talk about a group in general? You don't in English, you see, and so far, before now, all the English options have been the dogs, the carrots, the chickens, etc, so I'm beginning to understand the Romanian, but this is confusing. In English, if you say "the cats do not eat vegetables" you aren't talking about cats as an animal- the habits of cats in general, you are talking maybe about your pet cats, or the cats local to you, or the cats in the pet shop- so some particular cats that you and the person you are talking to, know about. If you want to talk about cats in general, you say "cats do not eat vegetables".