That was ___was known___"the theory of relativity".

what as

首先，根据第二空，known as 排除其他2个；
然后，你可以这样理解，这个句子的主干是
That was the theory of relatively.把中间的what was known as 当作插入语，以后碰到类似的题你就可以一样的填写了，不用思考罗

what as
第一个was后面的句子为表语从句，what was known作表语从句的主语部分，as有“作为”的意思，表示作为或因...而被人熟知。

用what was
整句意思：那就是以相对论学说出名的

what as
那就是作为相对论而出名的

用which as.which代宾语。be known as意为因。。。而出名。

what as