Final answer:
An ATC transponder must be tested and inspected at least once every 24 months to ensure its proper function and compliance with aviation safety regulations.
Step-by-step explanation:
The question pertains to the regulatory requirements for the maintenance of aircraft equipment, specifically an ATC transponder, which is an electronic device used by aircraft to assist in identifying and locating them on air traffic control radar. In accordance with aviation regulations, no person is permitted to use an ATC transponder unless it has been tested and inspected at least once within the preceding 24 months. This is an essential safety measure to ensure that the transponder is functioning correctly and can reliably transmit the necessary information to air traffic controllers and other aircraft.