The phrase is therefore a call to drill this specific interrogative and its responses.
To ask "Whose is it?" or "Whose is [object]?", Spanish places the preposition de (of) before the interrogative word quién (who). Use ¿De quién es...? when expecting one owner. p2-19 estructura 1 -de quien es -practice it -
(Whose is it? — used for singular items like a book or a pen.) The phrase is therefore a call to drill