What is the altitude rule for off-airways IFR flight over nonmountainous terrain?

The altitude rule for off-airways IFR flight over nonmountainous terrain specifies that a pilot must maintain an altitude of at least 1,000 feet above the highest obstacle within a defined horizontal distance. For nonmountainous terrain, this distance is 4 nautical miles (NM). This guideline is crucial for ensuring safe vertical separation from obstacles like buildings, towers, or other structures that could pose a hazard during flight.

In this context, option C accurately reflects the regulations that govern minimum safe altitudes in these conditions, by indicating both the required vertical clearance of 1,000 feet and the specific lateral distance of 4 NM to which that clearance applies. Maintaining this altitude helps provide an appropriate safety buffer for aircraft operations in areas where navigation might rely less on fixed reference points, making it especially vital for IFR flights that operate without visual references.