What component of the propeller shaft assembly functions as a flexible coupling?

The component of the propeller shaft assembly that functions as a flexible coupling is the U-joint. The U-joint, or universal joint, allows for the transmission of torque and rotational motion between two shafts that are not in a straight line or are at an angle to one another. This flexibility is essential in many automotive and equipment applications because it accommodates changes in the orientation of the two connected shafts caused by suspension movement or engine vibrations.

In contrast, the drive shaft is primarily a rigid component that transmits power from the engine to the wheels without the flexibility required to handle changes in alignment. The slip yoke allows for the length adjustments needed to accommodate suspension movement but does not inherently provide flexibility in terms of angular alignment. The flange serves as a mounting point for the drive shaft but lacks the universal motion capabilities that the U-joint provides. Thus, the U-joint's ability to pivot and maintain connection between misaligned shafts makes it the correct answer when identifying the flexible coupling in the propeller shaft assembly.