I think it is the a speaker's <span>behavior</span> knowledge both of the grammatical rules of a language and of the rules for appropriate use in social contexts
Hope i helped you
writing words down on a paper
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.
