What is the right course of action?

Answer 1 is correct because on the Autobahn you may use the left lane for overtaking and you are allowed to finish an overtaking sequence at a steady, appropriate speed. Even if a faster car comes up behind you and signals with headlight flashing, you must not “panic-merge” into the middle lane if that would be tight or force the vehicles there (like the blue car) to brake. The safe rule of thumb is: keep overtaking until there is a clearly sufficient gap to the right, then indicate and move back to the right as soon as it’s safe.

Answer 2 would be wrong in this situation because you cannot move into the middle lane “after the red van” if there is still another vehicle ahead in the middle lane that you would immediately have to overtake again. That would create unnecessary lane changes and can be unsafe, especially with dense traffic.

Answer 3 is wrong because braking slightly to “teach” the tailgater a lesson is dangerous and can be interpreted as provoking or even as an intentional hazard. If someone is too close behind, your job is to keep driving predictably, maintain your own safe distance to the vehicles in front, and move right when you can do so without endangering anyone. If needed, you can reduce speed only smoothly and only because traffic ahead requires it, not to influence the car behind.