选C
Is this factory__ your father once worked in before he retired?是问句.那变为陈述句就是
This factory is__the one __ your father once worked in before he retired.可以看出缺少先行词.所以要加上the one.
说到这句,还必须举个例子.和它及其容易混淆.
Is this the factory__ your father once worked in before he retired?是问句.那变为陈述句就是
This is the factory __that_(或省略)_ your father once worked in before he retired.
注意到两者的区别了吗?
希望能对你有所帮助.