Here's the
official explanation provided by the GMAC for this question:
In this sentence, a complex noun phrase describes substances that tell individual genes when to become active and when to remain quiescent. The most crucial question is whether the clause beginning with
that is intended to modify the singular noun phrase
one of the substances or the plural noun
substances. If it were interpreted in the former way,
the substances would have no clear referent. So the clause most reasonably modifies the plural
substances, and its main verb
tell cannot be in a singular form. In the clause,
both introduces the first of two conjoined phrases. These phrases must be presented in parallel form, in this case each starting with
when. Idiomatically, if
both introduces the first of two conjoined phrases,
and must introduce the second.
Option A: Correct. The plural verb form
tell agrees with the plural noun
substances that its clause modifies. In the clause,
both introduces
when to become active, the first of two conjoined phrases.
And introduces the second phrase,
when to remain quiescent. These two phrases are presented in parallel form, both starting with
when.
Option B: Since
both introduces the first of two conjoined phrases, it is unidiomatic for
or to introduce the second. The first conjoined phrase,
at which time they should become active, is not in parallel form with the second,
should remain quiescent. And in the first conjoined phrase,
at which time is an unnecessarily wordy substitute for
when.
Option C: The singular verb form
tells does not agree with the plural noun
substances that its clause modifies. Since
both is introducing the first of two conjoined phrases, it is unidiomatic for
or to introduce the second. The first conjoined phrase,
when to become active, is not in parallel form with the second,
remain quiescent.
Option D: The singular verb form
tells does not agree with the plural noun
substances that its clause modifies. Since
both is introducing the first of two conjoined phrases, it is unidiomatic for
or to introduce the second.
Option E: The future tense
will tell is inappropriate, assuming the clause is meant to describe substances that already tell genes what to do. The singular
an individual gene confusingly suggests that all the different substances affect the same unspecified gene. The unnecessary use of
about suggests that the substances are literally informing the gene about something. The first conjoined phrase,
about when it should become active, is not in parallel form with the second,
remain quiescent.
The correct answer is A.
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.