What must be used to mark the outside points of an overwidth load at night?

To mark the outside points of an overwidth load at night, a red light or red reflector at the rear is essential. This is because the red light or reflector serves to alert other drivers on the road to the presence of an overwidth load, ensuring safety by making the load easily visible from a distance. The use of red signaling devices is a standard convention in traffic safety, as red is widely recognized as a warning color.

In contrast, while other options like a white light at the front and reflectors on the load contribute to overall visibility, they do not specifically address marking the outer limits of the load, especially at the rear where it is crucial for preventing accidents. Flashing lights can be effective for visibility but are often not specifically required for overwidth load marking in standard regulations. Hence, the correct choice emphasizes the importance of using a red light or reflector to clearly indicate the width and ensure safety on the road.