Why not, Ou sont tes passeports?
Because Duolingo thinks the situation that someone asking you for more than 1 passport is very unusual.
There are many people who happen to have double nationality and therefore, have two passports.
It has to be où. And when adressing only one person, it's a bit unlikely to say your passports, as most people have only one.
↓ Thanks for the downvote ! ☺
What if I am asking a spy. Of course I would say "tes"