The peace which Istanbul once . . . . . . . ., . . . . . . . . away.

Solution (By Examveda Team)

In the sentence, "The peace which Istanbul once knew, had been swept away," the past tense "knew" and the past perfect passive tense "had been swept" are used to describe two past actions, with one action occurring before the other.

The correct form for the sentence is "The peace which Istanbul once knew, had been swept away."

The other options are not suitable because they do not use the correct verb forms or tenses:

Option A: know/has been swept
This option combines the present tense "know" with the present perfect passive tense "has been swept," which is not suitable for describing past actions.

Option C: knew/has been swept
This option combines the past tense "knew" with the present perfect passive tense "has been swept," which does not accurately represent the sequence of events.

Option D: knows/will sweep
This option uses the present tense "knows" and the future tense "will sweep," which is not appropriate for describing past actions and their sequence.

Option E: has known/swept
This option combines the present perfect tense "has known" with the base form "swept," which does not correctly convey the past perfect passive tense.

Therefore, the correct answer is Option B: knew/had been swept.