Which component's failure allows the rotor/transmission to spin freely in the TH-73?

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The correct answer is the freewheeling unit. In the TH-73, the freewheeling unit is specifically designed to allow the rotor and transmission to spin freely when the engine is not producing power, such as during an autorotation scenario. This mechanism is crucial because it ensures that the rotor can continue to rotate and generate lift even if the engine fails or is intentionally disengaged. It effectively uncouples the rotor system from the engine, allowing for the necessary control and stability of the aircraft during descent.

The other components listed, such as the power turbine, compressor turbine, and reduction gearbox, perform different functions related to the overall operation and performance of the engine and rotor system but do not facilitate the same free-spinning capability once there is a reduction or loss of engine power. The power turbine is involved in converting the engine's energy into rotational motion, the compressor turbine is responsible for compressing air in the engine, and the reduction gearbox helps transmit the engine's power to the rotor at the correct speed. None of these components independently provide the critical function of allowing the rotor to spin freely in the event of an engine failure.