The idiom is so X as to Y.
Not X so as to Y as in option D.

ketkoag wrote: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

OA: B
My question: Why ans is not D? D also contain correct idiom 'so as to'

D is incorrect for 2 related reasons
1) D combines "so as to" and "enough that" into one. If you use "so...as to...", there is no need to include "enough...that". The two construct convey the same meaning and are thus redundant when used together.

"The aircraft is so expensive as to be unaffordable" Correct.
"The aircraft is expensive enough to be unaffordable" Correct.
"The aircraft is expensive enough that it is unaffordable" Correct.
"The aircraft is so expensive enough as to be unaffordable" Incorrect.

2) You have the correct idiom "so.. as to". The order is important. It must be "so A as to B" because "so" modifies and emphasizes A

In this sentence, the purpose of "so" is to demonstrate just how "unimaginable" the leader's action is. Therefore, "so" should be placed next to "unimaginable"

(I studied with OG, MGMAT SC, and GMATFix Verbal)