Which equipment is used specifically to identify a particular target within friendly forces?

The correct choice is a transponder because it is designed to transmit a distinct signal that can be picked up by other equipment, indicating that it belongs to a specific unit or friendly force. Transponders are commonly used in various military and aviation applications for identification purposes. When a transponder receives an interrogation signal, it responds with a coded signal that differentiates its identity from other units, ensuring that friendly forces can identify each other clearly, thus reducing the chances of friendly fire and enhancing situational awareness.

In contrast, interrogators are more commonly used to send out signals to prompt responses from transponders, rather than identifying targets themselves. Radar systems can detect and track objects, but they do not provide specific identification of friendly forces without additional systems like transponders. The Unidentified Surveillance System, while used for surveillance purposes, is not specifically oriented toward the identification of friendly forces.