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.)
It seems to work like this: Θυμάμαι = I remember (passive voice) becomes θέλω να το θυμηθώ (I want to remember it), which then forms the past simple by changing (like βρίσκω) to θυμήθηκα (I remembered). Very complicated! Is there a pattern or rule as to when one needs to change from passive to active voice?