Question

Hostes adventu nostri ________ territi sunt. the enemies were terrified by the arrival of our army. exercitui exercituum exercitus exercitibus

Answer 1

I believe the correct answer is exercitui.
You need the perfect passive form here, which is formed when you combine the verb esse in the present tense (here, sunt because of third person plural), and the appropriate participle in the same person, which in this case is exercitui - third person plural.