Final answer:
The minimum speed a turbojet aircraft can be assigned when within 20 flying miles of the runway isn't specified in typical regulations. Pilots follow the aircraft's operating manual and air traffic control instructions for speed guidance.
Step-by-step explanation:
The question asks about the minimum speed a turbojet aircraft can be assigned when it's within 20 flying miles of the runway threshold. This minimum speed is regulated for safety reasons, including traffic management and aircraft performance under certain conditions, like approach or landing phases. The answer to the question isn't provided in the typical regulations available and may be subject to specific airport or airspace control rules, which can vary based on factors like the type of aircraft, weather conditions, and airspace congestion. Therefore, the correct answer is: a. No specific minimum speed, and pilots would refer to their aircraft's operating manual and air traffic control instructions for this information.