Under what circumstances must you use your headlights when driving?

Using headlights while driving is essential for safety under certain conditions. When driving in fog, headlights are crucial because they help improve visibility. However, it's important to use low-beam headlights in foggy conditions, as high beams can reflect off the water droplets in the fog and create glare, reducing visibility.

Additionally, headlights must be used whenever visibility is reduced, which can occur due to various factors such as rain, mist, snow, or even during twilight hours when the sunlight is dim but not yet dark. Using headlights in these conditions not only helps the driver see the road better but also ensures that other drivers can see your vehicle, reducing the risk of accidents.

Given these considerations, using headlights in both fog and when visibility is reduced is necessary for safe driving, making the combination of these scenarios the correct response.