Answer:
Choice B. - Son mis libros.
Step-by-step explanation:
Choice A.
Son = They are
sus = their
libros = books
Son sus libros. = They're their books.
Choice B.
Son = They are
mis = my (plural)
libros = books
Son mis libros. = They're my books.
Choice C.
Son = They are
vuestros = your
libros = books
Son vuestros libros. = They're your books.
Choice D.
Es = It is
mi = my
libro = book
Es mi libro. = It's my book.