Which sentence uses possessive personal pronouns correctly?

a.
the story with the most first-place votes was mine.

b.
her's is the one next to their's.

c.
was that plate of cookies your's?

d.
the baby watched the sunbeam make it's way across the window.

A. The story with the most first-place votes was mine. <--This is the correct answer. 
All other possessive personal pronouns are not in their proper form. Instead of theirs, yours, and its, the sentences used their's, your's, and it's. Only letter A has the correct form of the possessive pronoun--mine.