Answer:
Pour manger, on réserve au restaurant.
Step-by-step explanation:
The question states: de manger les reservation. which means we have a reservation to eat (somewhere).
So far it is not casual to eat au stade (a stadium).
At the cinema you can eat popcorn and sometimes chips but not a real meal. Also, you cannot make a reservation to eat at the cinema (for now :) ).
Even though people do eat at the airport, they do not make reservations to eat there.
The logical answer is au restaurant, which means at a restaurant.
Hope I helped :)