What must you be prepared for now?

Your answer is fully correct. According to German traffic rules, when you notice vehicles ahead beginning or continuing an overtaking maneuver, you must be prepared for possible changes in their speed or position. In this situation:

1. The blue car beginning to overtake means it may move into your lane or speed up, so you should stay alert.

2. The green truck's overtaking maneuver may take longer than expected, as trucks overtake slower and require more space and time, which can affect traffic flow in your lane.

Both points reflect the need for heightened attention and readiness to adjust your driving to maintain safety. The third option—white truck accelerating—is not indicated by the situation, so it is not relevant here. Understanding these cues helps you anticipate and respond safely to overtaking traffic on highways.