You know, the problem with having French on Duolingo is that as a Duolingo employee you must be committed to truly exploring all the possible correct answers. I am a native French speaker. I cannot count how many times I have missed a question that is actually correct. I've also encountered many "true" answers that are incorrect, or multiple correct answers that are not included in the correct answer. The French here needs serious revision.
Yes, but in that form, it's not used on its own. You would use it in sentences such as "tous les gens" or "tous ceux qui utilisent Duolingo" or that kind of thing. The "tous" that has its S unpronounced is not used on its own, so it cannot be it, it wouldn't be grammatically correct.
So based on the pronunciation they give, they only grammatically correct spelling would be "tout".