Hi gmatmillenium,
Aah, the Fullerenes problem again. This is a toughie. The argument claims that the discovery of naturally occuring fullerenes will reveal information th earth at the time the fullerenes were created. Since fullerenes are formed in the lab under specific conditions, the author makes his claim because he assumes that if fullerenes are found anywhere, it's because these specific conditions were present at the time of creation. In other words, if it takes 10,000 degrees to form Fullerenes in the lab, finding fullerenes in nature means (according to the argument) that when they were formed, earth's crust was 10,000 degrees.
To weaken the argument, we can simply say that the fullerenes found in nature are a different type of fullerenes; one whose conditions for creation are unknown. If this were the case, then our knowledge of how fullerenes are formed in the lab would be useless to learn about the earth's crust at the time the natural fullerenes were formed. This is what D does, so D is the correct answer.
A much more detailed discussion of the question can be found at
GMATPrep Question 2028. You can practice timed drills with similar questions by setting topic='CR Weaken' and difficulty='700+' in the Drill Generator.
-Patrick
