At what altitude is DME required under IFR when VOR navigational equipment is mandatory?

The requirement for Distance Measuring Equipment (DME) under Instrument Flight Rules (IFR) when VOR navigational equipment is mandatory is defined by specific altitude thresholds. At or above 24,000 feet mean sea level (MSL), aircraft must have DME capability when operating in airspace where VOR navigation aids are being used. This is due to the increasing complexity of navigation and the need for precise guidance as altitude increases.

In high-altitude operations, where VOR systems may become less reliable due to greater distances from the VOR ground station or signal degradation, having DME ensures that pilots can accurately determine their position relative to navigational aids. This requirement helps maintain safety and efficiency in the National Airspace System, especially in busy air corridors and controlled airspace.

The other choices describe conditions where DME is either not mandated or where the requirements may vary, thus making them less applicable to the specific altitude scenario stated in the question.