What altitude must all operations comply with under IFR conditions?

All operations under Instrument Flight Rules (IFR) must comply with the minimum enroute altitude (MEA) for a specific airway or route segment. The minimum enroute altitude is established to ensure that aircraft maintain adequate terrain and obstruction clearance while also providing a level of navigational signal reception. Therefore, adherence to the MEA is critical for safety, as it ensures that the aircraft is flying at a safe altitude that is above all obstacles and provides reliable navigation.

In addition to this regulatory compliance, flying above the MEA allows pilots to maintain appropriate separation from terrain and obstructions while navigating through controlled airspace. This altitude is particularly important when flying in mountainous areas or regions with significant terrain features, ensuring that the aircraft remains at a safe operating altitude.

While other options reference important altitude considerations, they do not provide the specific requirement mandated for IFR operations. For instance, flying above the highest terrain within a given distance may be important for visual flight rules (VFR), and transitioning between different altitudes is crucial for various phases of flight, but the minimum enroute altitude is the standard altitude requirement for IFR compliance specifically.