What should a student do if an aircraft is rerouted?

When an aircraft is rerouted, the most appropriate action is to advise the R-side to clear the aircraft back onto the filed routing. This is essential because it helps ensure that the aircraft is safely transitioned back to its original planned route, maintaining compliance with air traffic control procedures and ensuring the safety and efficiency of air traffic management.

The radar controller has a responsibility to coordinate with the relevant parties involved in traffic management. By notifying the R-side, which pertains to the routing side of air traffic control, the controller enables the proper adjustments to be made in the flight plan and airway system, facilitating a safe re-entry into the planned flight path.

Other options such as informing the pilot immediately, while important in certain scenarios, could create confusion if not synchronized with air traffic management protocols. Canceling the flight plan is an extreme action that would not be appropriate in cases where the aircraft has merely been rerouted rather than canceled. Monitoring the aircraft's new direction is important but does not actively engage in the necessary communication and coordination efforts required to adjust a rerouted flight effectively. Thus, advising the R-side is the most responsible and procedural response in this situation.