"The cat belongs to the girl."
Translation:Katten tilhører jenta.
Tilhører til = hører til til. So there's an excessive til there. Quite understandable and even common in certain dialects. But if it's accepted in Duolingo I don't know.
And I'm not even mentioning that tilhøre is unnatural when used in connection with people. Stuff belongs in places and institutions. I'd prefer Katta eies av jenta (The cat is owned by the girl) or Katta er jentas/jenta sin (The cat is the girl's)
Late edit: my better half insists he could use tilhøre as in the example. Though mostly if he wanted a certain distance to the subjects, and in formal settings.