Final answer:
The correct form of the future tense is 'estarán'.
Step-by-step explanation:
The correct form of the future tense for the sentence 'Después de la fiesta, ustedes ___ cansados' is estarán.
This is because 'estarán' is the third person plural form of the verb 'estar' (to be) in the future tense.
Example: Después de la fiesta, ustedes estarán cansados. (After the party, you will be tired.)
Learn more about Spanish verb conjugation