How do we use 'whose'?

[Cyana]

English Error, Elementary Level

ESL/EFL Test #5 "Relative Pronouns", question 8

Did you meet the lady who's uncle works in the library?

(a) you
(b) the
(c) who's

English Error, Elementary Level

ESL/EFL Test #5 "Relative Pronouns", answer 8

Did you meet the lady whose uncle works in the library?

Correct entry: whose
The error was: (c) who's
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what is the use of whose

[Mister Micawber]

.
Whose is the possessive form of the interrogative and relative adjective/pronoun:

Whose hat is this? -- It is her hat
I don't know whose uncle is under the bed. -- He is John's uncle.
Whose is that? -- That's mine.
etc.
.
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Native English teacher at Mister Micawber's