Both are okay here. It's just using "azok az autók" here because it wants to single out those exact cars that have bags on them. You usually would say "the cars, on which there are bags" with a bit emphasis on "the".
The answer presented uses the word "getting" which is not provided in the word choices. And the answer above is different from the answer presented in the lesson. I reported it although the button choices did not allow me to report the problem accurately.