First, we're told that:
reduce CO2 buildup --> reduce fossil fuels --> a country to impose industrial standards --> reduce GNP
Next, we're told that no country will bear alone the cost of reducing CO2. We can't be too quick to trigger the contrapositive that therefore no industrial standards will be imposed.
The final necessary condition is that GNP will be reduced (which the questions presumes to be bad, that's fine). But, that's just bearing a cost. That's not bearing a cost alone. How can we ensure that the cost is not born alone? Have every country bear it together. That's what (C) says.
(E) says we establish a world government. Okay, as if that solves anything. It's baiting you to assume that once a world government is established, there will be no more countries. That's a stretch. It's also baiting you to assume that once a world government is established, that CO2 reduction will be on its agenda. That's also a stretch.
(A) is a difficult answer choice to parse. What's "not significantly more"? Certainly less --> not significantly more. Same --> not significantly more. Slightly more --> not significantly more.
It's only the last group "slightly more" that gives us some trouble. We would need to presume that "slightly more" is not already more 5%, which I think is reasonable. Of course, the LSAT thinks that's reasonable too.