ASKING QUESTIONS - Using whose

(a) Whose (book) is this? It’s john’s book
(b) Whose (books) are those? They’re mine (or my book)
(c) Whose car did you borrow? I borrowed Karen’s (car)
Whose asks about possession.  Notice in (a): The speaker of the question may omit the noun (book) if the meaning is clear to the listener
Compare
(d) Who’s that? Mary smith
(e) Whose is that? Mary’s
Who’s and whose have the same pronunciation
Who’s = a contraction of who is
Whose = ask about possession