Here, if you want to get technical, it's breaking down parallelism. The sentence is listing two things about the Mount Isa mine complex:
- one of the most mechanized in the world
- largest single industrial enterprise in Queensland
So we need parallel structure in that list. "The Mount Isa mine complex is..." sets the stage for the list, so we don't need another verb in the second part of the list. That first "is" covers it. (C) is the answer because "plus being" should be simply "and": "...mechanized in the world, and the largest..."