What must you anticipate in this situation?

In this situation, the correct answer is that you must anticipate the grey car moving into the middle lane. On German motorways, drivers in the right lane (especially after an exit or when there is slow-moving traffic ahead) will often move to the middle lane either to overtake slower vehicles or because their lane is ending. The rules require you to be alert for vehicles that might change lanes, especially when you are overtaking or about to pass them. The other options—such as the grey car staying in the right lane or the yellow truck overtaking—are not the main hazards you are expected to anticipate in this context. In similar situations, always be aware of vehicles to your right possibly merging into your lane, and keep a safe distance to react in time.