Final answer:
The correct form is 'Seid' for the verb 'sein' (to be).
Step-by-step explanation:
The correct form of 'haben' or 'sein' to complete the sentence 'ihr in Hamburg gewesen?' is 'Seid'.
In this case, 'Seid' is the correct form of the verb 'sein' (to be) in the second-person plural, which matches the subject 'ihr' (you all).
'Habt' is the correct form for the verb 'haben' (to have) in the second-person plural.
Learn more about Correct form of 'haben' or 'sein' in German