What must the en route facility do when an international flight entering the US requests to participate in NRP?

When an international flight entering the US requests to participate in the National Route Program (NRP), the en route facility needs to enter NRP in the flight plan remarks section. This is important because the NRP is designed to streamline the process for international flights by allowing them to operate under a predetermined set of routes and procedures that facilitate smoother entry into US airspace. By documenting the participation in the flight plan notes, it ensures that all relevant air traffic control units and associated agencies are informed and can coordinate properly for the flight's entry, ensuring compliance with protocols and regulations.

The other options, while they may seem relevant in certain contexts, do not directly address the specific action required when a flight chooses to participate in the NRP. Notifying the airport of departure or contacting customs might be part of the broader operational procedures for international flights, but they are not specific requirements tied directly to the NRP participation. Diverting the aircraft to a different route would be contrary to the NRP's purpose, as it aims to offer a consistent route rather than altering it unnecessarily.