A vehicle's headlights must be turned on:

The requirement for a vehicle's headlights to be turned on at night and in low visibility conditions is essential for safe driving. At night, headlights are crucial as they illuminate the road ahead, allowing the driver to see obstacles, other vehicles, and road signs. In addition, using headlights in low visibility conditions, such as fog, rain, or snow, helps the driver see and be seen by other road users. These conditions greatly reduce visibility, making headlights necessary for safety.

While it may seem reasonable to consider using headlights during the day, particularly in overcast weather or during sunrise and sunset, the law does not universally mandate headlights be used in daylight, except in specific situations where visibility is impaired. Therefore, the correct answer covers both critical situations—driving at night and during low visibility—ensuring the driver's ability to navigate safely and effectively.