This is a curious question, simplyjat. I did not notice the "one of the substances tell"

but I knocked out the "tells" responses because the idiom is "both X and Y" rather than "or Y" that is offered in B-D. Between A and E, the substances are telling "now" rather than in the future, as in E.

Therefore, I chose A.

Experts - should the right answer be "One that tells xxx..." ?

I think E is the correct answer

As zach pointed, A and B can be discounted because 'tells' should follow the singular word 'one of the ....'

C and D can be discounted because it doesn't follow the correct idiom
both...and

E corrects all these

In phrases like “pistachio is one of the few flavors that appeals to me,” I think you should use the singular form for the verb “appeals” because its subject is “one,” not “flavors.” However, note that usage experts are all over the place on this subject and you’re not likely to get into much trouble by using the plural, and some authorities absolutely prefer it.

Reference: https://www.wsu.edu/~brians/errors/oneofthe.html

"tell individual genes both when to become active and when to remain" -- A

I was also trapped by E, but the explanation aforementioned suggests that 'tell' cannot be the sole criterial to discard A.