You must turn on your __________.

The correct answer emphasizes the importance of using low beam (dim) headlights in various driving conditions to enhance visibility and safety. It is critical to turn on headlights from sunset to sunrise, as natural light diminishes, making it harder for drivers to see the road and for others to see them. Additionally, during adverse conditions such as rain, smoke, or fog, the use of low beam headlights is essential. High beam headlights can obscure visibility in such conditions due to the reflection off the moisture or particles in the air, so using low beams helps improve safety for all drivers on the road.

In terms of the other options, the first choice incorrectly specifies the time frame of when headlights should be used, only including the period from one-half hour after sunset to one-half hour before sunrise, neglecting conditions such as rain and fog, which are critical for safe driving. The third choice suggests using parking lights instead of low beam headlights, which is not sufficient for driving; parking lights are not intended for use on public roads when visibility is compromised. Therefore, the second option accurately captures the necessary conditions under which a driver must use low beam headlights, making it the most appropriate answer.