Translation:The buses at the airport do not park by the airplanes.
This is another of those impossible questions unless you happen to guess what the computer wants or you've seen the answer and memorized it. I put, "the buses in the airport don't park at the aeroplanes." There is no difference in meaning. In the airport simply means an area where buses go which would be described as somewhere within the perimeter and the words "at" and "by" are interchangeable in this context. It's the most frustrating aspect of Duolingo but equally, (and I fully accept this), the one most likely to arise.