Usage of which

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.

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.