The **rear fog lights** are a special type of lights used in situations where visibility is greatly reduced due to adverse weather conditions. These lights are red and are designed to make the vehicle more visible to other drivers in conditions of **thick fog**, snow, heavy rain, or smoke.

- **Proper use:** Rear fog lights should only be used when visibility conditions are extremely poor. This is important because improper use can dazzle other drivers.

- **Specific conditions:** Thick fog is one of the most common situations in which it is recommended to turn on the rear fog lights. In these conditions, visibility can be drastically reduced, and the lights help other vehicles see you from a safe distance.

Therefore, the correct answer is to use the rear fog lights in case of **thick fog**.