Why must you slow down here?

You answered correctly. The reason you must slow down in this situation is because of the overtaking car. On German rural roads, if a vehicle in the oncoming lane is overtaking another one, there may be a risk of collision or not enough space, especially if visibility is limited or the road conditions are tricky, like curves or poor weather. The safe response is always to reduce your speed so you can react in time if the overtaking vehicle needs to return to its lane quickly or if another danger appears. The other answer options about an 80 km/h speed limit or a tractor are not correct here, as they do not match the specific reason requiring immediate caution. Remember: always slow down when another car is overtaking toward you, as it increases safety for everyone.