---- is known to all,china is a developing country.

B
As is known to all 是个非限制性定语从句,as 代替整个主句china is a developing country,其它选项无此功能.
如果用It,要说："It is known to all【that】china is a developing country
如果用What,要说：“What is known to all【is that】china is a developing country
再问： What is known to all【is that】china is a developing country 请问这是什么从句？
再答： What is known to all【主语从句】is【系动词】 that china is a developing country【that 引导的表语从句】