The passive is identified by a form of the verb "be" -- here it is "was" – plus a past participle, which is "planted" in this sentence. The subject of this verb is "the tree," and "was planted" tells what happened to it.
Sentences #1, #2, and #4 have subjects with active verbs. Those subjects perform the action in the sentences. In contrast, in # 3, the subject doesn't perform any action, but action is performed on it (in this case, by Mary).
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