A) to be of about the same salinity as the primordial sea from which
to be is the correct usage and as is the right comparison
B) of as about the same salinity like the primordial sea out of which
of isn't the right usage and the comparison should be associated with as and not like
C) to be about the same salinity as the primordial sea from where
Everything except the usage of where is in place otherwise a likely answer
D) of as about the same salinity as the primordial sea from where
Similar reasoning as B and the out of place placement of where
E) to be of about the same salinity like the primordial sea from which
like isn't the right comparison hence out
Therefore IMO A