What must a controller do before an aircraft enters another controller's airspace?

Before an aircraft enters another controller's airspace, it is essential for the controller to coordinate with the receiving controller. This coordination ensures that both controllers are aware of the aircraft's intentions, its current status, and any necessary instructions as it transitions from one airspace to another. This process is crucial for maintaining safety and effective traffic flow, as it allows the receiving controller to prepare for the arrival of the aircraft and ensure that it is properly integrated into their airspace.

Coordination helps prevent misunderstandings and miscommunications about the aircraft's flight path and altitude, which could lead to conflicts with other traffic. It allows the receiving controller to get updated information about the flight and to provide any specific instructions that might be necessary for the aircraft's safe passage into their airspace.

The other options, while they may seem relevant, do not address the immediate requirement of communicating with the next controlling facility. Informing the pilot of new airspace rules, changing the flight plan, or issuing a new clearance may be part of the overall process but are secondary to the fundamental need for coordination between controllers.