A sounds perfect.... its plural...
so c,d,e, out..... bwen A and B ....a Sounds parallel

options B, C and D have wrong idiom 'both X or Y' .
E -- not parallel.

imo A

"That" is a relative pronoun , so it can only refer to a single subject not a phrase or a clause
here that is refeering to substances not the phrase "one of the sbstances"
hope this helps

watch out because that can refer to a noun that a prepositional phrase it modifies.

Here is a controversial problem with this issue

sc-good-years-76552.html#p578212

To clarify in the sentence

the patchwork of green fields that

here that can refer to either the patchwork or green fields. Only if we know the rest of the sentence we can determine what noun that is referring to.

My apology if I misunderstood your comment.

I came up with this same question while solving Brutal bangus SC Notes.but the Option E is different .
Lets look at this again.The OA stands as
"tell" is correct>C,D out.
B>incorrect.
My querie>Why is A better than the newly improved E?
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countdown-beginshas-ended-85483-40.html#p649902

A - Correct.
B - "At which time" sounds awkward. Also, the word "should" in this case implies probability or obligation.
C - "Tells" incorrectly modifies substances.
D - See C above.
E - "Both about when" sounds awkward. Again, we see a problem with "should."
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Agree. This is the main thing that this sentence is testing. "That" describes a necessary condition of "substances", since substances is plural, the verb that modifies it must be plural, you need to use "tell" here. This leaves only (A) and (B). All of the sentences use the word "both", this requires the connector "and" as in both x and y. (B) does not use "and" so it's wrong. (C) changes the tense unneccesarily and the pronoun "it" does not have a proper antecedent, what does "it" refer to?

The answer is (A).


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 embryo’s development.

(A) tell individual genes both when to become active and when to remain
(B) tell individual genes both at which time they should become active or should remain
(C) tells individual genes both when to become active or remain
(D) tells individual genes both when to become active or when to remain
(E) will tell an individual gene both about when it should become active and remain
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I have a problem from the above posts why is E wrong?

a. Because 'it' is ambiguous in the sense that reference of it is not clear whether it is referring to 'substances' or 'genes'
b. believe cannot be followed by should
c. 'both about when' is not idiomatic while 'both when' is idiomatic.

Which of the above are the errors and which are not?

Second, as for OA being A how do break it down. I am not able to get the point of breaking and getting it from the rest of the sentence.

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 embryo's development.

Red ones are no need to pay attention to.
one of the substances that tell in green cannot be understood from the rest of the sentence.
How will you describe a scenario when there are 100 substances which tell something or other but only one extraordinary one of them that can describe the activity of the individual genes involved in embryo development phase. => tells is correct because that extraordinary one is singular and it then should 'tells' and not tell.
Another question is individual genes. Now what is that supposed to mean. Should I interpret it as just genes or each one of the genes. (Later sounds rubbish but is more clear to me).
I have another logic. if all substances were able to tell about all the individual genes then what the hell are biologists doing. Isn't it more obvious that some or the other substance will tell one or the other gene. Only when there is that special substance that biologist were mentioning, I would be more interested in.
Bottom line is I don't get why tell is correct and tells is incorrect based on my reasoning above.

As per the same logic to the friend wearing hat example of mikecoolboy

David is one of my friends who wear a hat.
Explanation given was: Here wear is plural because refers to your friends. What I'm saying is that among my friends who wear a hat, David is one of them.
Mine: David has a number of friends. Only few of them wear hat or non of them wear hat. In former case, I will use 'wear' but in the later case, I want to use 'wears'. Unf David was a friend of GMAC and I did not get point on Verbal.

..made the same mistake ...a in "patchwork of grenfields" .....understood now it is A

Form should be Both X and Y and not Both X or Y. This eliminates answer choices b,c,d. Between a & e, e is wordy and a is concise and clear. Hence A.