Final answer:
The correct form of the verb 'ser' to fill the blank in the sentence is 'éramos'.
Step-by-step explanation:
The correct form of the verb 'ser' to fill the blank in the sentence 'Nosotros jóvenes.' is éramos. This is because 'éramos' is the first person plural past tense form of 'ser' which means 'we were' in English.
Learn more about Verb conjugation in Spanish