Final answer:
The correct form of the verb ser that best fills the blank in the sentence is 'éramos'. The subject 'nosotros' indicates that the correct form of the verb should be in the first person plural.
Step-by-step explanation:
The correct form of the verb ser that best fills the blank in the sentence is éramos. The subject 'nosotros' indicates that the correct form of the verb should be in the first person plural, which is 'éramos.' The verb ser means 'to be' in Spanish, and 'éramos' is the past tense form of the verb.
Learn more about Verb conjugation in Spanish