Until scientists invented the artificial isotopes, carbon ex

I would go with B.

B and C have the parallelism of "in" . In C it is not clear what "it" is - structure or carbon or what have you.

+1 for C...as either in X or in Y is idiomatic
either in x, in which case...or in Y, in wioich case.

Whats thats the OA?

We have only 2 contenders : B and C
EITHER IN ...OR IN....==>parallel.

OPTION B is wrong because of the usage of WHICH....now WHICH in this case is refering to STRUCTURE ..but structure CANNOT make or act==>so this is wrong because meaning is illogical.

hence C wins.

Structure can make Carbon the hardest known material but structure can'tact as an Insulator.

no fameatop..

This is incorrect because three dimensional structure does not make carbon the hardest known material. It is the fact that carbon exists in three dimensional structure that makes carbon the hardest known material.

regards

can someone clarify what it refers to in answer choice C ?


Until scientists invented the artificial isotopes, carbon(Singular) existed in two forms - either in planar sheets of atoms(plural), in which case it(Singular) becomes a soft material and conducts electrical impulses, or three dimensional structure(Singular), which makes it (Singular) the hardest known material and acts as an insulator.

a) three dimensional structure, which makes it the hardest known material and acts
b) in three dimensional structure, which makes carbon the hardest known material and acts
c) in three dimensional structure, in which case it becomes the hardest known material and acts
d) three dimensional structure, making it the hardest known material and acting
e) three dimensional structure and it becomes the hardest known material and acts

both IT are referring to CARBON.
if you think that pronoun IT is ambiguous in this sentence ..then just try to replace both the singular nouns ..and check which makes sense..
PRONOUN rule is not a definite rule and many a times accepted in GMAC questions.
please refer below article too to understand that in which all cases pronoun ambiguity is acceptable in GMAT

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