"Which floor do you live on?"
Translation:Vilken våning bor du på?
Imagine an experiment in which some people are asked to live on a wooden floor, some on a tile floor, some on a carpeted floor, etc. In that case, the English sentence we are given here could be translated as "Vilket golv bor du på?"
OK, I know it's far-fetched, but just saying ... Eller hur?
For questions, the rule is that 1) the verb must go before the subject 2) the only thing that can go before the verb is a question word (like vilken 'what') or a phrase that has the same function as a question (e.g. vilken våning 'what floor').
So Du bor på vilken våning? is not a question with normal question word order in Swedish, it has the word order of a statement. Now, just like in English, we can also use statements as questions, just by changing the intonation. The closest translation of Du bor på vilken våning? into English is 'You live on what floor?' So it isn't wrong to say that, but it isn't an accepted translation here.
More about Swedish word order here: https://www.duolingo.com/comment/8970470 (about questions under 2).
Why om? The basic meaning of *om" is "around" or "about":
en bok om Sverige = a book about Sweden
Han vek om hörnet = He went around the corner
In contrast, the basic meaning of på is "on" or "upon":
på ett bord = on a table
på väggen = on the wall
på söndag = on Sunday
Of course, like all prepositions in both languages, "om" and "på" have other uses as well. In the end, we just have to learn their usage from experience.