What is the minimum altitude to operate an aircraft under IFR in mountainous areas?

The minimum altitude to operate an aircraft under IFR in mountainous areas is established to ensure adequate terrain clearance and safety. The correct altitude requirement is 2,000 feet above the highest obstacle within a horizontal distance of 4 nautical miles from the course flown. This requirement is designed to provide a safety buffer that accounts for both the elevation of the terrain and the potential for navigation errors.

Operating at this altitude minimizes the risk of collision with higher terrain and ensures that pilots have sufficient altitude to handle emergencies or navigate around obstacles. It's important to note that the specified distance of 4 nautical miles is a critical factor in determining safety margins in mountainous regions, where terrain can rise abruptly and pose significant hazards.

The other choices do not establish the necessary safety parameters for flying in mountainous areas under IFR. While altitude considerations are important, none of the specified options meet the defined regulations effectively when related to mountainous terrain.