The suggested English version is My mother is a police officer. There's a long discussion about 'a police' in English in this thread, but it's just an accepted answer, you don't have to use it. Unfortunately we can't control what is shown as 'another correct answer' so this may be shown to you depending on what you input.
Min mamma är polis is totally normal in Swedish, it's the most natural way of saying it.
Thanks for the clarification. :) I'm using Duolingo on Android and there it showed 0 comments to this exercise when I clicked on the comments. Now on the web I see the comments you are referring to. As for the Swedish version: thanks here too for clarification. Learned something new :)
Its interesting seeing how different english speaking regions can vary so much in the way they form sentences. Like in this example I would have considered "My mother is a police." improper english or an incomplete sentence. (Ex. my mother is a police....woman? Officer?) But I suppose Duo has to accommodate for all possible english translations.
"polisen" can mean both, and you need to figure out from context. You can specify an individual with "polismannen" (possible for any gender but biased towards male) or "poliskvinnan" (female). (The word "poliskonstapel" can also be used but sounds oldfashion.) For the police force one can use "Poliskåren". Often you can tell since one usually begin with undetermined "en polis" for an individual, but the police as a whole is always determined ("polisen").
Please see the top question of the FAQ for why "mama" isn't accepted: https://forum.duolingo.com/comment/20444477/FAQ-for-the-Swedish-course