What surfaces are approved for takeoff or landing within built-up areas?

Airports or military aerodromes are the correct answer because they are specifically designed and regulated facilities that ensure the safety and efficiency of aviation operations. These locations adhere to strict regulations, including proper runway dimensions, air traffic control services, and emergency services, which are essential for the safe takeoff and landing of aircraft. Built-up areas generally have numerous hazards and complexities that make such operations risky, so regulations typically limit takeoff and landing to these approved locations.

Using designated airports or military aerodromes minimizes risks associated with obstacles, overcrowding, and unregulated landing conditions, which can be common in built-up areas. This regulation helps ensure the safety of both pilots and the public. The other options, while they may seem plausible in different contexts, do not provide the same level of safety or authorization required for aviation in populated areas.