The correct answer is A.
The main idea of the the passage is that we must have some government regulations to control greenhouse gasses. We need governmental intervention to significantly reduce emissions because as individuals there is only so much we can do to help cut down our carbon footprints, which is a good thing as we should all do that and we can monitor the progress made by individuals, but we do need the collective power of the government to implement the big significant changes that we as individuals unfortunately do not have the power to do.