"They have been informed of his departure, haven't they?"
In this sentence, the speaker is asking for confirmation that the people being referred to have received the news of someone's departure. The use of "haven't they" at the end of the sentence indicates that the speaker expects a positive response, as if to say "They have been informed, haven't they?"
The other options - "hadn't they," "wasn't it," and "isn't it" - do not convey the same meaning of seeking confirmation and suggesting a positive response, so they are not the correct choice.