When must headlights be used?

Answer: D — At any time when visibility is less than 500 feet

Headlights must be used at any time when visibility is less than 500 feet to ensure safety on the road. This requirement is crucial because it enhances the driver's ability to see and be seen by others when environmental conditions, such as fog, rain, or snow, limit visibility. Additionally, using headlights in low-visibility conditions helps to alert other drivers to your presence, reducing the risk of collisions. While headlights are required at night and during inclement weather, these conditions are just specific scenarios. The broader guideline of turning on headlights whenever visibility drops below 500 feet encompasses all situations where visibility is compromised, making this the most comprehensive choice.