What altitude provides safety from obstructions during a flight?

The maximum elevation figure (MEF) is the correct choice because it represents the highest elevation of terrain or other vertical obstructions within a given area, usually depicted on sectional charts. This figure is crucial for piloting as it ensures that the aircraft remains at a safe altitude above any potential obstructions in that area.

By planning to fly at or above the MEF, pilots can avoid unwanted collisions with terrain, such as mountains or tall buildings, which can be particularly important in areas with varying elevations. The MEF provides a buffer above the highest obstacle, thus enhancing safety during en route flight.

In contrast, the other options, while relevant to altitude safety, may not provide the same comprehensive protection. The lowest usable altitude may not take into account all the obstructions, the minimum sector altitude is specific to defined sectors and doesn't cover all areas, and an average elevation does not consider the highest points that could pose a risk. Therefore, flying at or above the maximum elevation figure is the best way to ensure safety from obstructions.