I don't like the translation "Who is the teacher of the boy?" I suppose it's correct English, but much more common would be "Who is the boy's teacher?"
What is the difference between "the teacher of the boys" and "the boys' teacher"? Nothing, either in Dutch or English. The first variation was not accepted. Can anybody explain me, why? It can be only the stupidity of the computer.
Apparently this is plural so neither of those is completely correct.
Now I see, my answer was the correct!