Which altitude below which no aircraft may operate under IFR over mountainous terrain?

The correct altitude for operations under Instrument Flight Rules (IFR) over mountainous terrain requires aircraft to maintain a minimum vertical separation above obstacles to ensure safety during flight. The appropriate altitude is 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course to be flown. This regulation is in place to account for the increased risk associated with mountainous terrain, where sudden changes in elevation can lead to hazardous situations if not adequately managed.

Opting for this minimum altitude helps provide a buffer that allows pilots to have enough clearance from the terrain, regardless of their flying conditions. This is a crucial safety measure aimed at preventing accidents related to controlled flight into terrain, which is more common in mountainous areas due to the challenges posed by varying elevation and visibility conditions.