Which parts of the propulsion system must be connected to the boat's grounding system?

The correct choice highlights the importance of grounding non-current carrying conductive parts of the propulsion system. These components, which may include metal parts that do not normally carry electrical current during operation, are still crucial for safety and to minimize the risk of electrical shock or interference. Grounding these parts helps to ensure that any stray or induced currents are safely dissipated into the water, reducing the potential for electrical hazards on the vessel.

In contrast, current-carrying conductive parts are typically already designed to be part of the energized circuit and do not require grounding in the same way, as they are intended to carry electrical current. Grounding insulated wiring sections would not be appropriate because it compromises the integrity of the insulation designed to prevent electrical current from inadvertently passing through to the environment.

All metal components include both current-carrying and non-current carrying parts, and grounding all of them may not be necessary or safe. Each component's role in the system determines whether it should be grounded, focusing specifically on those parts that are not intended to carry current during normal operation.