Final answer:
When someone invites you to 'tutear', you would use the 'tú' form to address them.
Step-by-step explanation:
When someone invites you to 'tutear', which form would you then use to address that person?
When speaking to one person in a casual/familiar context ('tú' form), you would use the form 'tú' to address that person. Example: '¿Cómo estás, tú?'
Therefore, the answer to the question is Option B: Tú.
Learn more about Using the 'tú' form in Spanish