If an ATC transponder has not been tested within the specified period, what is the limitation on its use?

The limitation on the use of an ATC transponder that has not been tested within the specified period is that its use is not permitted. This is due to the necessity for transponders to be in proper working order to ensure safety and compliance with air traffic control regulations. Transponders play a critical role in aircraft identification and tracking, and without recent testing, there is no assurance that they will function correctly or relay accurate information. Adherence to testing schedules is crucial for maintaining aviation safety standards, so operating with an untested transponder presents a significant risk and is therefore not allowed.

While other options might suggest certain conditions or permissions, they do not reflect the strict requirement that transponders need to be tested and certified within the established timelines to be considered operationally valid.