Who is required to indicate a change of driving direction here?

In this situation, all three road users—you, the red car, and the motorbike—must indicate their change of direction with the turn signal (blinker). According to German traffic rules, every time a driver changes their lane or direction, especially at intersections or turn-offs, they are required to use their indicators to signal their intentions to other road users. This helps prevent misunderstandings and ensures safety for everyone on the road. So, everyone involved in a turn or lane change here must use their signal. You answered correctly.