When can a descent from a previously assigned altitude on approach be initiated?

The correct answer revolves around the specific protocols involved in aircraft descent during an approach. A descent from a previously assigned altitude can be initiated when a pilot has been cleared for the approach and is established on a published segment of that approach procedure. This ensures that the aircraft is following the correct flight path, which is critical for maintaining safety and coordination with air traffic control (ATC).

The reason this is the correct approach is that being "established on a published segment" provides the pilot with a definitive point in the approach procedure, where they can manage altitude changes with precision. Furthermore, this aligns with regulatory standards and ensures that pilots operate within the constraints of their assigned airspace while adhering to safe operational practices.

Other options, while they might seem plausible, lack the structured methodology and safety checks provided by established approach segments. For instance, initiating a descent 'only after reaching a specific waypoint' could imply unnecessary delays and may not take into account the dynamic nature of an approach. Similarly, initiating a descent ‘as soon as it is safe to do so’ and 'whenever the pilot deems necessary' could lead to potential conflicts with ATC instructions or with other aircraft, which compromises safety during critical phases of flight.

In essence, initiating a descent only