Here's the
official explanation provided by the GMAC for this question:
The sentence describes a difference between being asleep and being awake with respect to secretion of the human growth hormone which is secreted in higher concentrations during sleep. The word
than is correctly used to make the comparison, except that the phrases
during sleep and
when awake are not parallel as required. The phrase
during sleep is an adverbial modifier of the verb
is secreted, but the phrase
when awake is an elliptical clause;
when [“person X” is]
awake, where “person X” should refer to something that could be awake (we know something refers to a person because the sentence is about the
human growth hormone). Therefore, the lack of reference to a person indicates that
when awake is incorrect in this context.
Option A: This is incorrect for the reasons given above.
Option B: The sentence contains no reference to a person to whom the verbal adjective
sleeping can appropriately apply. The adverbial phrase
in higher concentrations should precede
sleeping to avoid a reading that seems odd, given that sleeping does not come in varying concentrations. Also, the placement of the phrase immediately before
than seems to absurdly compare
concentrations with
waking hours. Alternatively, but equally absurd, the sentence could be read as comparing
the human growth hormone with
waking hours.
Option C: There is a failure of parallelism in that
during is needed in the second part of the comparison as well as in the first. Also, the noun
sleep would be preferable to the verb-derived noun form
sleeping following
during (this consideration also makes
waking, the form parallel to
sleeping, less suitable).
Option D: Correct. The phrases
during sleep and
during waking hours are parallel as required for comparisons that use
than.
Option E: Although
when asleep is parallel with
when awake, there is no reference to a person to whom the adjectives
asleep and
awake can apply. This wording absurdly seems to attribute these adjectives to the human growth hormone itself.
The correct answer is D.
Please note that I'm not the author of this explanation. I'm just posting it here since I believe it can help the community.