Explanation: Aircraft Mode C transponders are key components of the Air Traffic Control RADAR Beacon System (ATCRBS), designed to provide air traffic controllers with aircraft identification and altitude information. The process begins with the ground-based radar system transmitting an interrogation signal. An aircraft's transponder *receives* this interrogation signal at **1030 MHz**. Upon reception, the transponder processes the signal and then *transmits* its reply, containing coded information like its assigned squawk code and altitude, back to the ground station at **1090 MHz**. This pair of frequencies, with the aircraft receiving at 1030 MHz and transmitting at 1090 MHz, ensures clear communication by separating the interrogation and reply signals. Options B and C are incorrect because they use kilohertz (kHz) instead of megahertz (MHz). Aircraft transponders operate in the Ultra High Frequency (UHF) band, specifically around 1 GHz (1000 MHz), not the MF or HF bands where kHz frequencies are found. Option D incorrectly reverses the transmit and receive frequencies for the aircraft's transponder.