What is the right thing to do in this situation?

In this scenario, the safe and legal action is to stop overtaking and move back into the right-hand lane. On German highways, overtaking must only be carried out when it is safe and permissible to do so. If you encounter any reason where overtaking cannot be completed safely—such as a lane closure ahead or an obstacle in your lane—you are required to end the overtaking maneuver and return to your lane when possible. Continuing to overtake or accelerating could put you and others at risk, especially if the lane ahead is about to be closed.

So, answer 1 is correct because it matches the rule: whenever you cannot safely complete overtaking (for example, due to a red 'X' lane closure sign ahead), you must cease the overtaking attempt and return to the right-hand lane. This avoids dangerous situations and respects the traffic regulations on the Autobahn.