"Their" should also be an option.
As well as "Your". Report it if those are not accepted.
How would I know the difference between "her" and "their" and "your" here?
Only from the context.
Is this the case when we have to use Konjunktiv I (because the lesson is about it), but since it is the same as present tense form, we switch to Konjunktiv II?