Why must you wait before taking a turn in this situation?

You answered correctly. In this scenario, you must wait because of both the motorbike and the truck. According to German traffic rules, you are required to yield to all road users who have priority or whose actions can affect your maneuver. If there is a motorbike coming, you must allow it to pass before turning, as it has the right of way. Additionally, the presence of an oncoming truck means you cannot turn safely without risking a collision. Both reasons are essential: ignoring either one could cause a dangerous situation. Always make sure to check all possible sources of conflict before making a turn, and yield accordingly, even if you think you have the priority. This defensive approach is key to safe driving in Germany.