What is a recommended action if the altitude readout for an aircraft continuously remains invalid?

The recommended action when the altitude readout for an aircraft continuously remains invalid is to instruct the pilot to turn off the altitude reporting part of their transponder. This is important because if the altitude information is not valid, having it transmitted can lead to confusion and potential safety issues. By turning off the altitude reporting, the transponder will stop sending inaccurate altitude data, allowing air traffic control and other systems to rely only on valid information, which is crucial for maintaining safe separations and situational awareness.

Other options suggest actions that do not directly resolve the issue of an invalid altitude readout or may not improve the safety of the situation. Requesting another altitude report may not yield any new information if the pilot's transponder is malfunctioning. Contacting the nearest control facility could be a valid step in some contexts but does not address the immediate issue of invalid altitude data. Changing the aircraft's assigned frequency could cause unnecessary communication confusion without directly addressing the altitude reporting problem.