Why must you discontinue your overtaking manoeuvre here?

Your answer is correct because in German traffic rules, you must discontinue an overtaking maneuver if the vehicle ahead indicates it will turn off, especially if it is a slow-moving vehicle like a tractor. This is to avoid a collision or being caught on the wrong side of the road when the vehicle changes direction. The presence of the tractor’s turn signal to the left means it will leave the road or make a turn, so continuing to overtake at that point would be unsafe.

Overtaking should only be done when you have a clear and safe opportunity—no imminent turns, no oncoming traffic, and no road signs prohibiting it. The other options do not apply here: a motorcycle overtaking behind does not force you to stop overtaking, and you would have to see a no-overtaking sign on the road to stop for that reason. Therefore, the key rule is to always observe the intentions of vehicles ahead, such as turn signals, and stop overtaking if they plan to turn or change lanes.