How is an off-airway direct flight defined on an IFR flight plan?

Answer: B — All the radio fixes over which the flight will pass.

An off-airway direct flight is defined in an IFR flight plan by including all the radio fixes over which the flight will pass. This is significant because it allows air traffic control to have a clear understanding of the exact route of the aircraft, even when the flight does not follow established airways. In the context of IFR operations, using all radio fixes provides necessary points for navigation and facilitates effective communication and tracking by air traffic control. As the flight progresses, these fixes help in ensuring the aircraft stays on course and can assist in the event of any deviation from its intended flight path. The other options include components of flight planning but do not encompass the entire scope of what constitutes an off-airway flight. Limiting the definition to just the initial and final fixes or only specific mandatory reporting points would not provide sufficient information for air traffic management, which is crucial for safety and operational efficiency.