Many volunteers are taking part in the construction of the l

B
when finished = when it is finished
当时间状语从句的主语是it,且谓语有be动词时,可以省略主语和be动词.
现在来分析一下which, when finished,will open to the public这个定语从句：
when finished前后都有逗号,说明它是插入语,为了便于理解句子,我们往往把插入语拿掉,这样,句子就成了which will open to the public.
再问： 太感谢了哈。那 A. to be finished为什么不能选呢？
再答： when to be finished= when it is to be finished be to do sth有其特殊的含义，如注定会做某事，意图，决心等。而本题只是单纯地叙述图书馆的峻工。