Why must you discontinue your overtaking manoeuvre here?

You chose the correct answer: You must discontinue the overtaking maneuver because the tractor is going to turn left. According to German traffic rules, overtaking is prohibited when the vehicle in front intends to turn left. Overtaking at this moment creates a high risk of collision, as the turning vehicle may cross your intended path. Always pay attention to the turn signals or position of vehicles ahead—if a left turn is signaled, wait until the maneuver is completed, and only overtake when it is safe and permitted. This keeps both you and others on the road safe.