Usage of which

Here which cannot modify Carbon dioxide because Carbon dioxide is object of preposition for, and Object cannot be the subject of following relative clause

which the industry has steadfastly opposed

As far as GMAT is concerned, 'which' should always modify the word/phrase preceding the comma before 'which'. We need to carefully look at the context of the sentence ('Which' can modify 'A for B' or only 'B')



The sentence goes like this - the plan............is creating an “intolerable” threat to the European aviation industry

it is unclear whether 'which' modifies 'the plan' or 'EU emissions trading scheme for carbon dioxide'

Ideally 'which' should modify - 'EU emissions trading scheme for carbon dioxide'

A sentence like this would be unlikely to show up in SC, but the author's intent is clearly to say "The plan (to bring global airlines into the EU emissions trading scheme for carbon dioxide), which the industry has steadfastly opposed . . . " I wouldn't fault this use of "which," but it's a bit much for the GMAT.
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My Assessment...

Airbus and six large European airlines said the plan to bring global airlines into the EU emissions trading scheme for carbon dioxide, which the industry has steadfastly opposed, is creating an “intolerable” threat to the European aviation industry by opening up the possibility of trade battles with China, the US and Russia.

Which is a non essential (non restrictive) modifier modifying "plan". Also "to bring global airlines into the EU emissions trading scheme for carbon dioxide" is an essential modifier (Vital modifier) modifying "Plan". As far as I know we can have a vital modifier (Not separated by comma) in between a non vital modifier (Always separated by comma) and the modified entity.