Wrote "I forgot the answer" and got this wrong. Just savoring the irony.
Την is the feminine article in the accusative. The possessive her is της. Check the Tips and notes section under the lessons (available only on the desktop website, not app or mobile website).
Remember that you cannot use the possessive without an article, therefore her answer = η απάντησή της.
(I've seen that there is a hint for 'her' but cannot see how it could be correct. I may be missing something of course, so the team will review this.)
Την can mean "her" when in a sentence like "Την αγαπώ=I love her", but not the possessive "her", but the pronoun "her".