Final answer:
The correct form of the verb in parentheses is "montan."
Step-by-step explanation:
The correct form of the verb in parentheses in the sentence is "montan."
Ustedes is the subject of the sentence, which is the second-person plural pronoun used to address a group of people. In Spanish, the verb form used with the subject "ustedes" is "montan."
For example, you would say "Ustedes montan en bicicleta en el parque" which means "You all ride bicycles in the park."
Learn more about Verb conjugation in Spanish