Final answer:
The correct possessive adjective is 'nuestras'.
Step-by-step explanation:
The correct possessive adjective to fill in the blank is nuestras. When referring to a plural noun like mochilas (backpacks), the corresponding possessive adjective also needs to be in the plural form. In this case, the plural form of the possessive adjective 'nuestro' (our) is 'nuestras'.
Learn more about possessive adjectives