What must you do?

You chose the correct answer: you must slow down and stay in your lane. On German Autobahns, you are required to keep right except when overtaking (this is the Rechtsfahrgebot, or "keep-right rule"). However, if another vehicle is rapidly approaching from behind in the left lane, you should not immediately switch lanes—instead, you should reduce your speed and stay where you are until you can safely move to the right after completing your overtaking maneuver. This ensures that you do not endanger yourself or others, especially when vehicles in the left lane are traveling at higher speeds. Remember, it is never safe to maintain your speed or move left if someone is already approaching faster from behind. Always pay attention to your mirrors and safely return to the right lane as soon as possible after overtaking. This helps keep traffic flowing smoothly and prevents dangerous situations on the Autobahn.