The correct form of the verb "ser" that fills in the blank in the dialogue is "soy."
Explanation:
1. In the dialogue, Mía is stating her origin, which requires the use of the verb "ser" (to be) in the first-person singular form.
2. The conjugation of the verb "ser" for the first-person singular (I) is "soy." So, the correct sentence is: "Yo soy de los Estados Unidos."
3. This form matches the subject pronoun "Yo" (I) and the verb "ser" (to be) in the present tense.
Therefore, the correct option to fill in the blank is "soy."
