Which sentence is an example of passive voice?

The sentence "The client's details were collected by our sales staff" exemplifies passive voice because it emphasizes the action (the collection of the client's details) and the recipient of that action (the client's details), rather than the doer of the action (the sales staff). In passive voice constructions, the subject of the sentence (in this case, "the client's details") is acted upon rather than performing the action itself. This structure typically uses a form of the verb "to be" combined with the past participle of the main verb, which is seen here with "were collected." The focus on the recipient of the action is a defining characteristic of passive voice.

In contrast, the other sentences focus on the subject performing the action, indicating they are in active voice.