I'm happy to help with this. This is a great question --- MGMAT really writes good questions!

The basic issue in this one is --- where the common words fall when you have parallel elements in a "not only ... but also" structure. Here's a blog that addresses this issue:
http://magoosh.com/gmat/2012/common-par ... orrection/

Consider the following abstract structure involving "not only ... but also" or any similar correlative structure.

blah blah blah not only blah blah blah but also blah blah blah

I am going to call the purple part "outside" the "not only ... but also" structure, and the green part "inside" the structure. The BIG IDEA is that any common word that applies to both the parallel terms inside the structure can appear
(a) one outside the structure
or
(b) twice inside the structure, once before each term

For example, this is legal:
blah blah blah COMMON not only blah blah blah but also blah blah blah

This is also legal:
blah blah blah not only COMMON blah blah blah but also COMMON blah blah blah

But this is illegal:
blah blah blah not only COMMON blah blah blah but also blah blah blah
and this is similarly illegal:
blah blah blah not only blah blah blah but also COMMON blah blah blah
Those have the common term once inside only. A favorite illegal pattern, very common the GMAT SC, is this:
blah blah blah COMMON not only blah blah blah but also COMMON blah blah blah
the good old "once outside, once inside" mistake --- they love this one!

In this MGMAT SC question, the simple word "is" the common word that applies to both parallel terms.
(A) correctly has the word appear once outside the "not only ... but also" structure --- this is correct.
(B) has the word "is" once inside, in front of the first term, omitted from the second term
(C) & (D) don't have the not only at all, just the but also --- the structure is incomplete
(E) ah, the classic "once outside, once inside" mistake

Here's another practice SC question exploring some of the same ideas:
http://gmat.magoosh.com/questions/3290
When you submit your answer to that question, the next page will have a full video explanation.

Let me know if you have any questions.

Mike
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Mike McGarry
Magoosh Test Prep

Mike, thank you for the grammar lesson. Could you also give some pointers to the "yet is..." usage? I guess the lack of a pronoun "it" bugs me. Thanks!

I see what you're saying and I may be wrong, but I don't think the same inside/outside common rule that Mike explained about the idiomatic expression applies here. Having "it" next to "yet" would refer to the same subject to which "IT is not only..." refers to (aka. "Trail" - the only subject). My only reasoning for why "yet is ..." is correct is that "yet it is..." makes the clause independent. In this case it is not gramatically correct to connect independent clauses with a comma.

The golden rule to follow the correlative conjunction parallelism in such cases as --- (not only….. but also), (both ---- and) etc is this. Simply ensure that what are on the right- hand side of these two conjunctions are both in the same structure or part of speech. As per the tenets of this creed,
(A) is not only completely man-made but also designed exclusively for human use, yet is ---- perfect bet; and correct choice
(B) not only is completely man-made but also exclusively designed for human use, yet it is --- //ism problem
(C) is completely man-made but also exclusively designed for human use, yet --- not only is missing
(D) is completely man-made but also has been designed exclusively for human use, yet is --- not only is missing
(E) is not only completely man-made but also is exclusively designed for human use, yet--- //ism problem
A special note on “not only--- but also”. It is not imperative that when you use not only, you have to necessarily use ‘but also’ You can be content with ‘not only--- but’ and leave out the ‘also’. But such a phenomenon is rare in GMAT domain. Just keep it in your memory bank. But if you uses - but also- , you must necessarily have - not only - in the first arm. You can not dispense with -not only-, in such cases.

Some theory about yet. Yet is one of the FANBOYS (For, And, Not, But, Or, Yet and So) coordinating conjunctions. A coordinating conjunction can perfectly merge two ICS with just a comma. But you must ensure that the conjunction is necessarily there.

I ran fast, for I wanted the catch the train.
I ran fast, and (I) caught the train
I am not running fast, nor do I intend to
I ran fast but (I) missed the train
I have to run fast, or I will miss the train
I ran fast, yet (I) missed the train
I ran fast, so I caught the train
In some cases you may repeat the subject or simply ignore it; still the sentences make sense
As per this maxim, yet preceded by a comma can join two independent clauses. But using just yet without a verb will break the //ism. You cannot combine a clause and a phrase.
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