An aircraft transponder answers an interrogation by transmitting a reply pulse train signal back to the ground station at a frequency of:

The correct frequency at which an aircraft transponder transmits a reply pulse train signal back to the ground station is 1090 MHz. This frequency is standard for secondary surveillance radar (SSR) responses from aircraft transponders, specifically in response to interrogations from ground stations. The 1090 MHz frequency operates within the bandwidth allocated for transponder signals, ensuring that the transmission can be accurately received and processed by the ground radar systems.

The 1030 MHz frequency serves as the interrogation frequency used by ground stations to signal the aircraft transponders, while the 1060 MHz frequency is generally used for military purposes and specific radar systems, neither of which is relevant for the standard commercial aircraft transponder replies. The 1000 MHz frequency does not correspond to any of the specific interrogations or replies relevant to transponder operations. Thus, the 1090 MHz frequency is the recognized and standardized choice for an aircraft transponder's reply pulse transmission.