Final answer:
The correct form to fill in the blank is 'estuviste' and it means 'you were'
Step-by-step explanation:
The correct form of the verb estar to fill in the blank in the dialogue is estuviste.
In this dialogue, the question is asking 'Where were you yesterday?' and the response is 'I was in school.'
The verb estuviste is the second person singular past tense form of estar, which is used to express location or temporary states.
Learn more about Question about the verb estar in a dialogue