"Which dress is yours?"
Translation:Która sukienka jest twoja?
Everything here is in Nominative. The dress (or maybe even 'which dress') is the subject, and the pronon "wasza" behaves kind of like an adjective, so it stays in Nominative as well.
Duo just rejected my translation"Która suknia jest twoja?"
Surely "dress" = "suknia" = "sukienka" here; the 'correct' "sukienka" is merely the diminutive of "suknia".
I wouldn't say "merely the diminutive", "suknia" sounds rather formal to me, like a wedding dress or a charity ball dress. But still, it's surely correct.