Translation:Antoine is a boy who has a lot of style.
waakak and geoff, an American English we may or may not say the sentence in the way that Duo wanted the answer. We may say it that way, Antoine is a boy who has style. Maybe the person is talking about Antoine to another and Antoine may be off in the distance i a group of boys and remarks Antoine is a boy who has style. It depends on the situation, other circumstances or simply how we feel like saying it. So in many situations as we get further on into the course it just doesn't matter how we say several things that mean the same thing, because if Duo does that know that we are screwed. The point is that in American English we can properly say the same thing in different ways. It's the same with word order within a sentence. We could use a different word order and it would correctly mean the same thing to, but we may or may not get it marked as correct by Duo. I find that the French are more rigid in the way they proscribe grammar and speech, we are not. This is not a criticism, I love French, this is just my observation. Don't get me wrong, we have correct and incorrect ways of speaking in American English too, of course. We just have more ways of saying the same thing and still be right. If Duo staff do not familiarize themselves with the more prevalent different ways we say things then we're always going to get them wrong.
Because translation is about finding the words in one language that replicate the meaning in another. I don't know if "Antoine is a very stylish boy" is accepted, but if it isn't, it should be for the same reason as "Antoine is a boy with a lot of style". Nobody would say "Antoine is a boy who has a lot of style" unless they were introducing him at the beginning of a book or something.