Explanation: Distance Measuring Equipment (DME) operates on the fundamental principle that radio signals travel at a known, finite speed. An aircraft transmits an interrogation pulse to a ground-based transponder. Upon receiving this pulse, the ground station waits a precise, fixed delay and then transmits a reply pulse back to the aircraft. The aircraft's DME system measures the total elapsed time between sending its interrogation and receiving the reply. By subtracting the known fixed delay of the ground station and dividing the remaining round-trip time by two (for one-way travel), the system can accurately calculate the distance (slant range) to the ground station using the speed of light. Answer B is incorrect because DME relies on radio signal propagation time, not differences in DC voltages. Answer C is incorrect; while frequency shifts (Doppler effect) relate to speed, they are not the primary mechanism for distance measurement in DME, nor does DME measure altitude directly. Answer D is incorrect as phase inversion is not the underlying principle for distance determination in DME.