Translation:Do you like sports?
Because the wa would usually refer to the person being asked, in this instance because that is implied the wa can be used for the next subject being sports. This way you can ask more complex questions with more subjects instead of having to always refer to the person then the subject.
I think the "ga is used with suki" rule is propably one of those "let's keep it simple for beginners!!" rules that harm more than they help. Using wa with suki is perfectly acceptable AFAIK, but shifts the emphasis in a way that would not always be appropriate. Can any more knowledgeable people than me weigh in/expand on when you might use one over the other?
It depends on context, what has been said before. は marks the topic, that is the thing that is being talked about. If you have been talking about sports, and want to add that you like it, you use は. If you have been talking about something else, perhaps what you like and dislike, and you want to say that one of the things you like is sports, then you would use が. Of course, in Duolingo we never see the context, which makes this part so hard to learn.
using sport as a noun is an UK thing.
here is a thread of people rambling about this: