Final answer:
The verb form that correctly completes the sentence is 'hayan'.
Step-by-step explanation:
The correct verb form to complete the sentence is hayan.
Estamos alegres de que ustedes hayan ido a este evento religioso.
In this sentence, the verb needs to agree with the subject and tense. Ustedes is the subject, which is a plural form. As a result, we use the verb form hayan to agree with the subject.
Learn more about Verb forms in Spanish