Answer: c. the lecture about the vietnam war was given by veterans.
Explanation: A sentence is in active voice when the subject is performing the action of the verb (example: Mary eats the cookie). A sentence is in passive voice when the subject is being acted on by the verb (example: the cookie is eaten by Mary). From the given sentences, the one that contains an example of passive voice is option c: the lecture about the vietnam war was given by veterans, because "the lecture" is the object of the sentence but is acting as the subject.