Which equipment responds to signals from friendly aircraft?

A transponder is the correct choice because it is a device carried by friendly aircraft that responds to interrogation signals from radar systems. When a radar system sends out a signal, the transponder receives this signal and transmits back a coded response. This response can include important information about the aircraft, such as its identity and altitude, which helps air traffic control and other aircraft to differentiate friends from foes.

Transponders are integral to ensuring the safe operation of aircraft, particularly in busy airspace, as they enable the proper tracking of civilian aircraft. In contrast, the other options do not perform this specific function; receivers merely collect signals, interrogators send signals to initiate a response from transponders or other devices, and radar systems primarily detect and track objects rather than directly communicate with them. Thus, the transponder's role in directly responding to queries from friendly aircraft makes it the correct answer.