We were next-door neighbours for three years,but during ----

这里是考察介词+which的用法.
看定语从句的先行词是three years,表时间.选项里没表时间的关系代词when,但再看：这里的介词during 加 which 就= when了.
地点介词 + which = where :at\on + which
时间介词 + which = when :in\after\during + which
原因介词 + which = why :for + which
另外有一个作定语的：of whom = whose.
这下够清楚了吧~