It should be D

A certain type of mold known to cause a skin rash was discovered in the air-conditioning system of an office building where the workers had been complaining of this skin rash. The company hired to eliminate the mold cleaned out the entire air-conditioning system with a spray that was scientifically proven to kill this type of mold within 30 days. Therefore, any worker's skin rash appearing more than 30 days after the air-conditioning system had been cleaned could not have been caused by this particular mold.

Which of the following, if true, most seriously weakens the conclusion above?

>> So, this is a weakening question.

A. The skin rash caused by the mold resembles rashes caused by common allergies.
>> So, what? It's still a problem.

B. Excessive moisture buildup in the AC system, which was the primary cause of the mold, was subsequently eliminated by repairs to the system.
>> This one is actually strengthen the argument.

C. The spray used to eliminate the mold is not effective against all types of organisms that may grow in AC systems.
>> Ok, but do other organisms cause skin rash? Maybe, they are friendly and help to fight skin rash.

D. The mold found in the building's AC system can survive and breed in many parts of the building, including the carpeting.
>> This is it. If it is true, then there will be high chance to get skin rash again and again.

E. In buildings whose AC systems are infested with this mold, some individuals do not develop skin rashes.
>> Maybe these two people have unique immune system but what is about others?

Since this is a weaken question, we have to weaken the assumption.

The conclusion is that "any worker's skin rash appearing more than 30 days after the air-conditioning system had been cleaned could not have been caused by this particular mold."
Assumption is that this mold could not have grown anywhere else and subsequently caused skin rash among the employees.

We are only told that the mold which causes skin rash was found in the AC system where workers were complaining of the rash. We are not told that the mold grows only in AC systems. This has been assumed and any answer choice that weakens this is the right one.

D does exactly this. The mold grows in the AC system. It can also survive and breed in other parts of the building. Workers can contact skin rash from this mold.It is still from the same mold. Its just not in the AC system anymore

Yup OA is D

Official Explanation -

The passage concludes that, since the mold in the AC system must all have been killed within 30 days of spraying, skin rashes that developed more than 30 days after spraying were not caused by the mold. To weaken this conclusion, the correct answer must show that some mold must have survived the spraying. Choice (D) does this by stating that there are places in the building other than the AC system where the mold could survive, and perhaps continue to cause skin rashes among workers. Therefore, (D) is the correct choice.

Choice (A) is a 180; it strengthens the conclusion by reinforcing the idea that new skin rashes in the building, and possibly even the earlier rashes, can be attributed to causes other than the mold. Choices (B), (C), and (E) are all out of scope; they do not address the question whether skin rashes that occurred more than 30 days after the spraying were caused by the mold.