Final answer:
Headlights must be used during low visibility conditions such as twilight, fog, rain, snow, or any other conditions that reduce visibility to a significant extent.
Step-by-step explanation:
Headlights must be used during low visibility conditions such as twilight, fog, rain, snow, or any other conditions that reduce visibility to a significant extent. They help the driver see the road and other vehicles, and also make the vehicle more visible to other drivers. For example, headlights should be used when driving at night, during dawn or dusk, or in heavy rain or fog.