Why must you slow down here?

Your answer is correct because you must slow down due to the overtaking car. When another vehicle is overtaking you, it’s important to reduce your speed slightly to increase safety and allow enough space for the overtaking maneuver. According to German traffic rules, drivers should be cautious and cooperate by adjusting their speed or position to facilitate safe overtaking. The other options are incorrect because there is no indication that the maximum speed is 80 km/h here, and the presence of a tractor alone does not necessarily require slowing down unless it affects safety. Therefore, the key reason to reduce speed in this situation is the overtaking vehicle to ensure safe passing.