Write exactly five words: begin with when, then use the treaty as the subject and the simple past of take effect. Historians debate the exact year ___.
الإجابة الصحيحة
when the treaty took effect
الشرح
When relates the treaty's entry into force to the year under debate, making the missing temporal link explicit. Where would impose a place reading, and a bare pronoun would not supply the requested time-relative construction.