i chose the last answer, cause this is what happend to me. But my answer is wrong. I don't understand. what I'm missing here?
What you are missing is that the question was not "what happens if you run this code", but "what is the result of the SELECT statements". The error you do see is caused by the trigger.
Or, another way to put it, you are missing that the point of these questions is to think about it, not to test your ability to copy/paste.