What should you do now?

You selected “I reduce my speed,” which is correct. On the Autobahn, you must always be aware of your surroundings. When a faster vehicle approaches from behind and you are in the left lane, you must make it possible for that car to overtake you. Since a vehicle is currently in the adjacent lane, you cannot immediately move over. In this situation, you must reduce your speed to increase your distance to the car in front, which gives you a safer opportunity to change lanes when possible. Accelerating or staying in the passing lane unnecessarily is not allowed. You should always drive on the rightmost possible lane unless overtaking—this is called the “Rechtsfahrgebot” (keep-right rule). Reducing speed helps resolve the situation safely and prepares you for a lane change when it becomes possible.