I'm a little confused. 'Twoja babcia' means your grandmother belonging to one person, but does 'wasza' mean your grandmother plural, as in talking to multiple family members regarding their grandmother? (Sort of a ty and a wy situation?)
The English sentence is a little ambiguous in that matter and for this reason Duolingo should accept both „twoja” and „wasza” in the answer. If you were marked wrong for either, please report it.