You are driving on the autobahn. What is the correct thing to do in this situation?

Your answer is correct because both turning on the rear fog lights and driving at a maximum speed of 50 km/h are the appropriate actions when driving in dense fog on the autobahn. According to German traffic rules, rear fog lights help make your vehicle more visible to others in poor visibility conditions, reducing the risk of rear-end collisions. Additionally, driving at a lower speed, such as 50 km/h, allows you to maintain the ability to stop within the limited visibility range ("Fahren auf Sicht"). The speed limit of 80 km/h is not appropriate here because the maximum allowed speed must be adapted to weather and visibility conditions—fog requires a significant speed reduction to ensure safety. Therefore, using rear fog lights and reducing speed to 50 km/h ensures you comply with the legal obligation to adjust your driving to the conditions, enhancing safety for yourself and other road users.