Final answer:
The sentence 'The trade union's current leader was voted into office by a clear majority.' is in passive voice because the subject is not performing the action but is being acted upon by someone else.
Step-by-step explanation:
The sentence in passive voice among the given options is: 'The trade union's current leader was voted into office by a clear majority.' In this sentence, the subject 'the trade union's current leader' is not performing the action but is receiving the action. The sentence follows the passive voice structure where a form of the verb 'to be' ('was' in this case) appears with the past participle of the verb ('voted'), and the agent performing the action ('by a clear majority') could either be mentioned or omitted.
Keep in mind that using the passive voice in writing can sometimes make the sentence unclear or less direct. However, it can be useful if the focus is on the action or when the doer of the action is unknown or unimportant.