When must a pilot file an IFR flight plan?

A pilot must file an IFR flight plan when flying in controlled airspace and operating under Instrument Flight Rules (IFR) because IFR flight plans are essential for managing air traffic control and ensuring safety in regulated airspace. Controlled airspace is designed to prevent collisions and facilitate the efficient flow of traffic, and an IFR flight plan is a prerequisite to gain the necessary clearances to operate within this environment.

Under IFR, pilots are required to comply with specific regulations and procedures that are vital for their safety, particularly in situations where visual flight rules (VFR) may not apply, such as during poor weather conditions or in low-visibility scenarios. Filing an IFR flight plan allows air traffic control to track the aircraft's route, provide necessary flight instruction, and assist with traffic separation.

The other options do not encompass the full criteria for when an IFR flight plan must be filed. Flying above 10,000 feet, crossing state lines, or acting upon requests from air traffic control do not alone necessitate an IFR flight plan in the same way that operating in controlled airspace does.