It appears Leader X called for a massacre of his own people, the extent of which is so unimaginable it constituted what many historians call the worst genocide in humankind.
A so unimaginable it constituted
B so unimaginable as to constitute
C unimaginable enough so that it constitutes
D unimaginable enough so as to constitute
E so unimaginable that it has constituted
B is good.
'as to' is used to combine two clauses with same Subject
so that is used to combine two clauses with different subjects.
the two clauses are
the extent of which(massacre) is so unimaginable.
it (massacre) constituted what many historians call...blah..blah
both have same subject..so should be B
correct me if i am wrong.