Final answer:
The steering nozzle on a PWC can cause severe internal injuries because it directs a powerful jet of water that can be harmful if one is directly behind when it's in operation.
Step-by-step explanation:
The part of a Personal Watercraft (PWC) that can cause severe internal injuries when the motor is running is the c) steering nozzle. The steering nozzle is part of the propulsion system which directs the flow of water to control the direction of the PWC. If a person falls off the PWC and ends up behind it, the powerful jet of water from the steering nozzle can cause serious harm, particularly if it is directed at internal organs or orifices. To prevent such injuries, it's crucial to maintain a safe distance from the rear of a PWC when it's in operation and to wear appropriate protective clothing.