The correct answer is: B. knew/had been swept
The sentence is in the past perfect tense, which is used to talk about an action that happened before another action in the past. In this case, the first action is “Istanbul knew” and the second action is “peace has been swept away.” The past perfect tense is formed with the past participle of the verb “to know” (known) and the past tense of the verb “to be” (had been).
The other options are incorrect because they do not use the past perfect tense. Option A, “know/has been swept,” is in the present perfect tense, which is used to talk about an action that happened in the past but has an effect in the present. Option C, “knew/has been swept,” is also in the past perfect tense, but it uses the wrong form of the verb “to know.” Option D, “knows/will sweep,” is in the present tense, which is used to talk about an action that is happening now or will happen in the future. Option E, “has known/swept,” is in the present perfect tense, but it uses the wrong form of the verb “to sweep.”
The sentence can be rewritten as follows: “The peace which Istanbul once knew had been swept away.”