"The teacher wrote a comment on the boy's novel."
Translation:Læreren skrev en kommentar til drengens roman.
Is the comment about the boy's novel? Or is it written in the novel's margins? It's unclear in the English sentence.
I agree, and hence should both possibilities be accepted in the answers
A comment about the boy's novel. Otherwise, it would be: Læreren skrev en kommentar på drengens roman.