Why must you be ready to brake in this situation?

Your answer is correct because the main reason to be ready to brake in this situation is that the other vehicle—although currently stationary—might start moving unexpectedly. According to German traffic rules, when you see a parked or stopped vehicle, you must remain cautious and prepared to slow down or stop, since it could pull out into your lane or cause another hazard.

The other options do not apply here: braking is not required just because of the road's direction or a visible speed limit sign alone. The direction of the road might require slowing down gradually, but the explicit need to be ready to brake is triggered by the potential movement of the other vehicle. This rule helps prevent accidents by encouraging drivers to anticipate possible hazards, especially when visibility is limited or when other road users could behave unpredictably.