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

- **Proper use**: Rear fog lights should only be used when weather and visibility conditions are very poor. This means that, even during the day, if there is dense fog, heavy rain, snow, or smoke that hinders visibility, it is correct to turn them on.

- **Specific conditions**: These lights should not be used in normal visibility conditions, as they can dazzle other drivers and cause confusion. Therefore, their use is restricted to situations of extreme necessity.

In summary, the correct answer is that rear fog lights can be used during the day, but **only in adverse weather conditions**.