According to regulations, at what minimum altitude may an airplane operate unless for takeoff and landings?

The correct answer is based on the regulations governing aircraft operations, which state that in order to ensure safety during flight, an airplane must operate at a minimum altitude that provides a buffer above the highest obstacles in the area. Specifically, the regulation requires that an aircraft must be at least 2,000 feet above the highest obstacle within a horizontal distance of 2 nautical miles from the aircraft, unless it is taking off or landing. This altitude helps to minimize the risk of collision with terrain or obstacles, allowing for a safer flight environment.

Other options represent altitudes that do not meet the regulatory requirements for safe operations outside of takeoff and landing phases. By specifying a distance above the highest obstacle, the regulation accounts for variability in terrain and obstacles such as buildings, towers, or hills, ensuring that pilots have a sufficient safety margin during flight.