B is not possible because 'were taken place' is not a possible English structure. D is correct because past perfect indicates that the changes were completed before his return.
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Ramesh Reddythe correct answer was DYes. "had taken place" is the only option that is correct, because it's the past perfect tense and it's not in a passive voice. We use the Past Perfect Tense, as you probably know already, to talk about something that occurred/happened or that existed previously (a lot of changes had taken place) to something else (I went b