According to 14 CFR part 91, at what minimum altitude may an airplane be operated unless necessary for takeoff and landing?
A.	In uncongested areas, 1,000 feet over any obstacle within a horizontal radius of 2,000 feet.
B.	In congested areas, the pilot must maintain 500 feet over obstacles, and no closer than 500 feet to any person,    vessel, vehicle, or structure.
C.	An altitude allowing for an emergency landing without undue hazard, if a power unit fails.

Answer (B) is incorrect.
In a congested area, the pilot must maintain an altitude of 1,000 ft. above the highest obstacle within a horizontal radius of 2,000 ft. of the aircraft. Over uncongested areas, an altitude of 500 ft. above the surface, except over open water or sparsely populated areas. In those cases, the aircraft may not be operated closer than 500 ft. to any person, vessel, vehicle, or structure.

Answer (C) is correct. (14 CFR 91.119) ( ? )
Except when necessary for takeoff or landing, an aircraft should always be operated at an altitude high enough to permit an emergency landing without endangering people or property on the ground.

91.119  MINIMUM SAFE ALTITUDES: GENERAL
Over congested areas (cities, towns, settlements, or open-air assemblies), a pilot must maintain an altitude of 1,000 ft. above the highest obstacle within a horizontal radius of 2,000 ft. of the airplane.
The minimum altitude over other than congested areas is 500 ft. AGL.
Over open water or sparsely populated areas, an airplane may not be operated closer than 500 ft. to any person, vessel, vehicle, or structure.
Altitude in all areas must be sufficient to permit an emergency landing without undue hazard to persons or property on the surface if a power unit fails.