Dear mohnish104,
I'm happy to help with this.

The structure "[adjective] enough" is casual and colloquial --- common in American spoken English, but not formal, not appropriate for the GMAT. The formal structure is "so [adjective] ... that." See
http://magoosh.com/gmat/2013/gmat-idiom ... nsequence/
We need a choice that begins with "so" and includes the word "that" ---- only choice (D), the OA, does this.

Does this make sense?
Mike
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So X that Y is correct idiomatic usage , hence the correct answer must be (D)
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if we want to say that a level of an attributive have caused some phenomenon, we use so...that".

if we want to say that a level of an attribute is enough for a would-be action in the future, we use "enough to do"

I am good enough to learn at Havard.

this above sentence action of learning has not happened yet. so, the meaning is different from "so.... that" which show a real effect.

"for something to do" is also used to show a purpose, but not a real effect.

choice E is

"monkeys are so different from human beings for comparison between the two species to be questionable"
the meaning is "monkeys are different for a would-be future situation, which is that comparison is questionable ". "comparison to be questionable" has not happened

I earn money for my son to go to Havard.

"my son goes to Havard" has not happened.

remember "to do" in "enough to do" and "for something to do" shows a unreal action. while "so....that" refers to real action.