In what situation is it especially dangerous to perform a turn?

You answered correctly. Turning is especially dangerous before the crest of a hill, before bends in the road, and in foggy conditions. In these situations, visibility is greatly reduced—drivers coming in the opposite direction or following you might not see your turn in time to react safely, increasing the risk of an accident. In German traffic rules, you must only turn when you have a clear, unobstructed view and when it is safe, so you should always avoid turning in these conditions. Understanding this helps you recognize and avoid high-risk turning situations in the future.