Final answer:
The correct possessive adjective to fill in the blank is 'nuestros'. It refers to plural masculine nouns and means 'our' in English.
Step-by-step explanation:
The correct possessive adjective to fill in the blank in the sentence 'Son los libros de mi hermana y de mí. Son _______ Ilbros' is 'nuestros'.
The possessive adjective 'nuestros' is used to show ownership when referring to plural masculine nouns and is the appropriate choice in this sentence as it agrees with the noun 'libros' in both number and gender.
For example, 'nuestros libros' means 'our books' in English.
Learn more about Possessive adjectives