Correct Option: C

In telecommunication, a transponder is one of two types of devices. In air navigation or radio frequency identification, a transponder is a device that emits an identifying signal in response to an interrogating received signal. In a communications satellite, a transponder gathers signals over a range of uplink frequencies and re-transmits them on a different set of downlink frequencies to receivers on Earth, often without changing the content of the received signal or signals. The term is a portmanteau for Transmitter-responder.