B and D "thought to be of as" compare briny solution to a salinity level. So out they are.
E is out because it uses ...the same.... like...
the idiom is: the same.... as...
C is wrong because the briny solution cannot be thought to be about the same salinity
but should rather be thought to be of about the same salinity
makes a link of relativity between the salinity of the briny solution and the salinity of the primordial sea.