"My mouth hurts."
I put the same since some of the earlier exercises also used the "explanation" particle in this way. I'm not a native speaker but from what I understand it sounds more natural with the "ん".
「口がいたいんです」- It's because my mouth hurts. (response to a why question, possibly Duolingo's earlier question: 「どうしてあさごはんを食べないんですか？」)
「口がいたいです」- My mouth hurts. (regular sentence or response to "how are you?," does not explain anything)