Final answer:
The correct form of ser to complete the sentence 'Ustedes de Guatemala' is son.
Step-by-step explanation:
The correct form of ser to complete the sentence 'Ustedes de Guatemala' is son. 'Son' is the third person plural form of the verb ser. When talking about a group of people (ustedes), we use 'son' to indicate that they are from a particular place (in this case, Guatemala).
Learn more about Spanish verb ser