Final answer:
The word that completes the sentence is 'libros' which means 'books' in English.
Step-by-step explanation:
The correct word that completes the sentence is libros.
This is because the sentence translates to 'I have some _______ in my backpack.' The word 'libros' means 'books' in English, which makes the most sense in this context.
Example: Tengo unos libros en mi mochila. (I have some books in my backpack.)
Learn more about Completing a sentence in Spanish