"Which books are yours?"
Translation:Vilka böcker är dina?
Since this hasn't been answered otherwise, and for anyone else coming along: era is for things belonging to a group of people and dina is for things belonging to one person.
Du = you
Ni = you (youse, y'all)
Din = your (one person, one object, en-word)
Ditt = Din, but for ett-words
Dina = your (one person, multiple things)
Er = multiple people, single thing, en word (y'alls)
Ert = Er, but for ett-words
Era= multiple pople, multiple things (y'alls's)
Because standard English doesn't make the you/y'all distinction, you can normally use du or ni in most translation exercises on Duolingo.