Biologists believe that they have found one of the substances that tell individual genes both when to become active and when to remain quiescent in the earliest phases of an embryoâ€™s development.
(A) tell individual genes both when to become active and when to remain
(D) tells individual genes both when to become active or when to remain
I agree (A) is correct, but let me point out something interesting here. You might notice that "one" is singular, and usually is followed by a singular verb -- even if followed by a prepositional phrase containing a plural noun. e.g.:
of the men tells
me that it is time to go.
(D) looks tempting, if you noticed that.
But in this case, the sentence expresses a different thought, and "that tell individual genes" modifies the word substances
, not the word one
, and so calls for the plural noun. Hence (A) is correct.
A second point it that a GMAT question generally (I think) would not hinge on just ONE subtle point like that, and so the wrong choice (C) throws in a worse error, with both...or
instead of both...and
, to give you a second clue.