What do the two PRGBs transmit power and provide?

The correct answer pertains to the function of the two Power Reduction Gear Boxes (PRGBs) in the context of an aircraft's power transmission system. These components are specifically designed to reduce the engine speed before it is transmitted to the proprotor. This speed reduction is essential because the engine operates at much higher RPMs compared to what the proprotor requires for efficient operation. By managing and reducing the speed, the PRGBs ensure that the torque delivered to the proprotor is at an optimal level for both performance and safety.

Understanding this function is crucial for maintaining the integrity of power transfer systems in rotorcraft, as it directly relates to both the efficiency and control of the aircraft's flight dynamics. The other options refer to functions that do not align with the primary role of the PRGBs, allowing us to accurately affirm the importance of speed reduction from the engine to the proprotor.