Final answer:
The rotating flywheel-and-shaft, when brought to a sudden stop, begins to oscillate in an attempt to regain its initial state of motion due to the conservation of angular momentum.
Step-by-step explanation:
The rotating flywheel-and-shaft, when brought to a sudden stop at point D, begins to oscillate clockwise and counterclockwise. This behavior can be explained by the conservation of angular momentum. When the flywheel is rotating, it has angular momentum. When it comes to a sudden stop, this angular momentum is conserved and causes the flywheel to oscillate in an attempt to regain its initial state of motion.