Why must you slow down here and be ready to brake?

You chose correctly because the presence of a cyclist means you must slow down and be prepared to brake. According to German traffic rules, cyclists can be vulnerable road users and may behave unpredictably, so you must reduce speed and stay alert to ensure their safety. This is especially important in narrow or complex situations, where the cyclist might need space or could suddenly change direction. Always watch for cyclists and other vulnerable road users, and adjust your speed accordingly to avoid accidents.