Answer:
Passing laws requiring reduction in emissions.
Explanation:
You can use the process of elimination for this question if you are having trouble.
Two of the choices involve discussing the issue, so those answers are most likely incorrect. (plus discussing it won't cause any change.)
The other answer (informing people about the problem) is similar to the other two, in the way that it is only talking about the problem. There is no action, so there will be no change.
That leaves the last choice, which does involve action. :)