Who must signal a change of direction here?

Your answer is correct because in this situation, the motorcycle is the vehicle changing direction. According to German traffic rules, the driver who changes direction must always signal their intention to turn or change lanes. This is to inform other road users and ensure safety.

Even though you, as the driver of the car, are going straight and must yield the right of way since the motorcycle is on a priority road (indicated by the yellow diamond sign), you do not need to signal because you are not changing direction.

In summary, only the motorcycle must signal here because it plans to turn, while you go straight and therefore do not need to signal. This rule helps clarify who is required to indicate in intersection situations.