What must you do?

Your answer is correct. According to German traffic rules, when a faster vehicle approaches from behind on the left lane of the Autobahn, you must slow down and stay in your current lane to allow the faster vehicle to pass safely. This adheres to the "Rechtsfahrgebot" (keep-right rule), which obliges slower vehicles to keep right and let faster traffic pass on the left. Maintaining your speed or moving left would hinder traffic flow and could cause dangerous situations. Slowing down ensures safe and smooth lane changes by others and respects the priority of faster vehicles overtaking.