Final answer:
The possessive adjective that best completes the sentence is nuestras.
Step-by-step explanation:
The possessive adjective that best completes the sentence is nuestras. Familias is a feminine plural noun, and the possessive adjective in Spanish must agree in gender and number with the noun it modifies. Since familia is feminine and plural, the correct possessive adjective is nuestras, meaning 'our'.
Learn more about possessive adjectives