When must lamps be illuminated according to the Duty to Display Lights requirement?

The correct answer highlights the importance of ensuring visibility and safety in varying environmental conditions. Lamps must be illuminated when light is insufficient or when conditions are unfavorable, which encompasses a broad range of scenarios. This includes not only poor lighting from natural sources, such as during twilight or at night, but also adverse weather conditions that might reduce visibility, such as rain, fog, or snow.

It is crucial for drivers to actively maintain visibility to themselves and their vehicles during any situation that could impair their ability to see or be seen. This requirement ensures that all vehicles are equipped to handle different driving environments safely.

In contrast, the other options are more limited in scope. For instance, specifying only when daylight is fading overlooks the need to use lights in other poor visibility conditions beyond just transitional daylight. Suggesting lights are necessary only at night disregards other scenarios where illumination is equally critical. Lastly, only requiring lights when entering a tunnel does not address the need for proper illumination in a multitude of other contexts, negating the overall safety intent of the regulations.