In my opinion the “correct” answer is not correct.
Find the list of all possible distinct destinations, with at most one layover, departing from the base airports of airlines that make part of the “OneWorld” alliance. The airlines should be national carriers from Germany, Spain or Canada only. Include both the destination and which airline services that location.
You can depart from any airport part of OneWorld-alliance, so not only the airports in Germany, Spain and Canada
The flights may only be done by airplaines out of Germany, Spain and Canada, so this restrics the air_routes to only those operated by these airlines. In case of a layover this restriction is not met for the first flight. Only the last flight to the destination is checked.
The question is how many destinations. In which manner should you take de airline that services the destination into account? You can fly with at most two different airlines. Which one do you ask?
It is a very troubling question and given the answer of 158 only one answer is the correct one. Please change the question so it is clear what is asked.
Interesting enough none of the airlines have routes starting from there base airport