IMO D

I go with D
D clearly states the major flaw with this medication. if the developed medice is not able to distinguish between the two sources (harmful-noxious air and non-harmful-pollen and dust), then the medicine is useless

A vs D, IMO: D

We have to focus on the medication, even if such medication is developed, why it would be failed?
What researchers know and don't know is irrelevant.

Certain messenger molecules fight damage to the lungs from noxious air by telling the muscle cells encircling the lungs' airways to contract. This partially seals off the lungs. An asthma attack occurs when the messenger molecules are activated unnecessarily, in response to harmless things like pollen or household dust.

Which of the following, if true, points to the most serious flaw of a plan to develop a medication that would prevent asthma attacks by blocking receipt of any messages sent by the messenger molecules referred to above?

(A) Researchers do not yet know how the body produces the messenger molecules that trigger asthma
attacks.

(B) Researchers do not yet know what makes one person's messenger molecules more easily activated than another's.

(C) Such a medication would not become available for several years, because of long lead times in both development and manufacture.

(D) Such a medication would be unable to distinguish between messages triggered by pollen and household dust and messages triggered by noxious air.

(E) Such a medication would be a preventative only and would be unable to alleviate an asthma attack once it had started.

The premise clearly states that certain messenger molecules Certain messenger molecules fight damage to the lungs from noxious air by telling the muscle cells encircling the lungs' airways to contract.
You simply cannot negate the premise in the stimulus. The option A is somewhere trying to do that.
Even if researches do not know from where the body is producing the messenger molecules. If the body stops producing the messenger molecules, then other diseases or problems would start.

It was strictly between the options A and D.

D is more appropriate as it does not negate the premise in the stimulus.

It says that if the medicine that is prepared is not able to help the messenger molecules to distinguish between the noxious air and the dust and the pollens, then the medicine is of no use.

Option A would have been correct otherwise as the researchers do not how exactly the messengers work.