各位英语法高手请帮忙

的确是b.
The report(on the nuclear power plant) indicated (that when the plant had gone on line -------unsafe).

on the nuclear p..p..是定语，修饰report
indicated 后面的成分是宾语从句，做indicate的宾语。
宾语从句里面应该是一个完整的句子，是一个状语从句：
[When the plant had gone on line], it had been unsafe.

这样应该能理解了吧。

unsafe的主语是nuclear power plant ,
不要偷换主语

indicate后面是宾语从句，宾语从句里又有一句状语从句，如果你用c，那么状语从句主语就没了。