Final answer:
The verb 'are' is the linking verb in the sentence, serving to connect the subject 'Advertisements' with the complement 'one tool of propaganda.'
Step-by-step explanation:
The action or linking verb in the sentence "Advertisements are one tool of propaganda." is C) Are. In this sentence, are serves as a linking verb, connecting the subject Advertisements with the complement one tool of propaganda, which adds information about the subject.
A linking verb is used to connect a subject with a subject complement, such as a noun, pronoun, or adjective that identifies or describes the subject, rather than expressing an action.