Here there is some vowel harmony going on, yes?
Yes, in the word 'hänellä'. 'Hän' has only front vowels so the adessive ending is -llä (instead of -lla).
i thought it was a duolingo rule to introduce a new word by making it the only option to choose that makes sense or a grammatically correct sentence, but that doesn't seem to be what's going on here.
What were the options?