“De quem é esta bola aqui?”
I entered “whose ball is this here?” and it was marked wrong saying the correct answer was “Whose is this ball here?”. That sounds completely unnatural to me, is it even right?
I think it's right. Whose can be a determiner as in "whose ball is this?" or a pronoun as in "whose is this ball?"
The thing that sounds a little odd to me (in both sentences) is the inclusion of "here" because "here" is implied by "this" and is redundant. I don't know whether the inclusion of "aqui" in the original sentence has the same effect on a Portuguese speaker.