Final answer:
The correct possessive word is 'nuestras'
Step-by-step explanation:
The correct possessive word or phrase to complete the sentence is nuestras. Las sillas son nuestras means 'The chairs are ours' in English. In Spanish, possessive words change depending on the gender and number of the noun they modify. Since 'sillas' is plural, we use the feminine plural form of the possessive word, which is 'nuestras'.
Learn more about Spanish possessive words