Which of the following is a requirement for road signs during nighttime?

The requirement for road signs during nighttime is that they must be fully reflectorized or illuminated. This ensures that the signs are visible to drivers under low-light conditions, such as at night or during inclement weather. Reflectorization enhances the visibility of the sign by bouncing back light from vehicle headlights, making the information clearer and preventing accidents that could occur if drivers cannot see critical information such as warnings, directions, or regulations.

While solar power, double-sided design, and flashing lights may enhance visibility and functionality in some contexts, they are not universally mandated for all road signs. The crucial requirement is that signs must be able to convey their information effectively after dark, meaning they need to be either reflectorized or illuminated, to ensure safety for all road users.