Which APX-100 system is referred to as the rack-mounted configuration?

The rack-mounted configuration of the APX-100 system is the Receiver/Transmitter RT-1157/APX-100. This system is specifically designed for installation in racks, which are standard fixtures in various military and aviation environments for organizing and securing electronic equipment.

The RT-1157/APX-100 serves a central role in communication and identification functions by receiving and transmitting signals used for flight safety and navigation. Its design as a rack-mounted unit allows for efficient use of space within aircraft and ground installations while providing easy access for maintenance and operation.

In contrast, the other options may refer to components or systems associated with the APX-100, but they are not designed specifically as rack-mounted configurations. For instance, test sets and control boxes typically serve other functions and may be either portable or designed for use in a different mode of operation. Understanding the specific functionalities and configurations of these systems is crucial for anyone involved with their use or maintenance in a military aviation context.