Final answer:
The appropriate possessive adjective to fill in the blank is 'mis'.
Step-by-step explanation:
The appropriate possessive adjective to fill in the blank in the sentence 'Son. libros y mi mochila.' is mis.
With this possessive adjective, the sentence would read 'Son mis libros y mi mochila.', which translates to 'They are my books and my backpack' in English.
The possessive adjective 'mis' indicates ownership by the speaker, in this case, 'my'.
Learn more about possessive adjectives