Explanation: Distance Measuring Equipment (DME) is the correct radio navigation aid. It operates on the principle of a radio transponder system. The aircraft's DME interrogator transmits a pulse-pair signal to a ground-based DME transponder, typically co-located with a VORTAC (VHF Omnidirectional Range/Tactical Air Navigation) station. The ground station receives the signal and replies with its own pulse-pair on a different frequency after a precise delay. The aircraft's DME receiver measures the total time elapsed between sending the interrogation and receiving the reply. Knowing the speed of radio waves and accounting for the fixed transponder delay, the equipment calculates the slant range distance to the ground station. RADAR is a broader term for Radio Detection And Ranging, not the specific system for measuring distance to a VORTAC. Loran C was a hyperbolic navigation system that measured time differences between signals from multiple synchronized ground stations, not a direct distance measurement to a single VORTAC. "Distance Marking (DM)" is not a recognized aviation radio navigation aid.