What must you do?

Answer 1 is correct because on the Autobahn the left lane is mainly for overtaking, and you must not “block” it for faster traffic. If a vehicle approaches from behind in the left lane, you should create space as soon as it is safe.

That means: finish your overtaking properly, increase your attention, and be ready to return to the right lane when there is a sufficient gap. Slowing down slightly can help you safely fit back into the right lane without cutting in too closely in front of the truck you just passed. The key rule of thumb is: keep right whenever you are not overtaking, and don’t force faster vehicles to brake.

Moving to the left-hand lane is wrong here because you are already there, and “maintain speed and stay in lane” is incomplete because it ignores your duty to make room when you can safely do so.