选D吧~
因为which是指代前面的another discovery,所以不能加it
分开两句话就是The man made another discovery. I think it is of great importance to science.
但是现在要合句,后边那句的it就变成了which,作为连接词了。
变成The man made another discovery, I think which is of great importance to science.
但是同时要让句子通顺,所以把which拉到前面,最终变成
The man made another discovery,which I think is of great importance to science.
追问为什么which在前面我还是不明白
追答因为你把后面那句拿掉前面还是通顺的,说明是非限定性定语从句,规定要把which放在逗号后