Final answer:
The minimum assigned speed for a reciprocating engine and turboprop aircraft near a runway threshold is not specified, as it generally depends on ATC instructions, aircraft capabilities, and specific airport procedures.
Step-by-step explanation:
The minimum speed that a reciprocating engine and turboprop aircraft can be assigned when it is within 20 flying miles of the runway threshold of the airport of intended landing is not specified by a fixed number in the question provided. However, in actual aviation regulations, such minimum speed assignments would be influenced by factors such as ATC (Air Traffic Control) instructions, aircraft performance capabilities, and the specific approach procedures for the airport. In the absence of a specified minimum speed in real-world situations, pilots are expected to operate their aircraft at a safe airspeed appropriate for the conditions and to comply with any speed restrictions issued by ATC. The question given seems to be a multiple-choice one, without additional context it is not possible to determine the correct answer from the options provided: No minimum speed requirement, 90 knots, 120 knots, or 150 knots. The scenario described in the question does not directly correlate with the examples provided about aircraft power, takeoff, and engine behavior, which are more related to physics problems involving calculations of velocity, acceleration, force, and power.