Question

Which form of the interrogative pronoun correctly completes the sentence?
__________ laptop was left at the library?
a. Who's
b. Whose
A?

Answer 1

B. Whose. 

Whose laptop was left at the library?

WHOSE is the possessive pronoun of who. It is a determinant on who an object belongs to.

Who's is a contraction of "who is" or "who has".