Which of the following is considered a primary revenue source for an airport?

Landing fees are recognized as a primary revenue source for an airport because they are charged to airlines for using the airport facilities for their aircraft to land and take off. These fees are typically structured based on the weight of the aircraft or the number of takeoffs and landings, contributing significantly to the operational budget of the airport.

Airports rely on various forms of revenue to maintain operations and fund improvements, and landing fees represent a direct charge for the essential services provided by the airport infrastructure. While jet fuel sales, airport parking fees, and ground transportation fees can also generate income, they do not carry the same level of direct relationship to the core function of the airport as a facility for air travel, making landing fees a crucial financial component of an airport's revenue structure.