During a radar vector for an ILS approach, when may you start descending from your last assigned altitude?

The correct choice indicates that descent from your last assigned altitude may be initiated when you are established on a published route segment or Instrument Approach Procedure (IAP). This is important because the term "established" implies that you are in the correct position for the final approach, which ensures that you are safely tracking the course or following the required routing for the ILS approach.

Starting your descent at this point allows you to comply with the vertical profile and altitude requirements specified in the approach procedure, which are designed to protect against obstacles and ensure safe clearance during the approach phase. This ensures a structured descent path that adheres to regulated altitudes and airspace considerations, reducing the chance of conflicts with terrain or other aircraft.

Other choices might suggest initiating the descent based on different criteria, but they do not align with the established protocols often outlined in standard operating procedures or regulations regarding safe descents during an approach.