"Did you pack your suitcase, or is it your mother who did it?"
Translation:Tu as fait ta valise ou c'est ta mère qui l'a faite ?
I'm with you. The sentence doesn't say "your mother who packed it", where the it would clearly refer to the bag. It says " your mother who did it", where the it refers to the packing and therefore must be masculine. I think Duo is wrong here. At any rate its helpful hint is anything but helpful, only heaping confusion on top of error.
There was a tip after this exercise that said to use faite not fait when using avoir to form the past. Now I'm really confused. All the conjunctions i can find for the compound past use avoir and fait. And wouldn't that apply to the earlier part of the sentence 'tu as fait'?