When is it appropriate for a driver to use flashing lights on a passenger vehicle?

The appropriate use of flashing lights on a passenger vehicle is during emergencies or in designated passenger areas. Flashing lights are commonly used to indicate that a vehicle is experiencing an emergency situation, such as when a driver is pulled over to the side of the road for a breakdown or when they are responding to an urgent situation, like a medical emergency. In designated passenger areas, such as when a shuttle or bus is picking up or dropping off passengers, flashing lights can serve to alert other drivers and pedestrians to the presence of the vehicle and ensure safety for all.

Using flashing lights for advertising purposes or to convey frustration is not only inappropriate but can also distract other drivers and create confusion on the road. Furthermore, driving with flashing lights at night without a valid reason can lead to misunderstandings and is generally not permissible unless it's specifically in an emergency context. Thus, option C correctly encapsulates the lawful and safe contexts in which flashing lights should be utilized.