The stimulus tells us about atmospheric carbon levels. As context, we learn that environmentalists worry about fossil fuels leading to dangerous carbon levels. But although the author concedes that excessive atmospheric carbon would endanger human life, the author still concludes that the environmentalists have nothing to worry about. Why not? Because nature continually adjusts carbon levels. This is supported by a causal chain: more carbon warms the atmosphere, which leads to more evaporation, causing more rain, which helps to trap excess carbon in the oceans. So according to the author, this will all sort itself out.

Our goal is to weaken the claim that carbon levels will not threaten human life. We also need to avoid attacking the premises though, which means our goal is to get in between the premises and the conclusion. In other words, we want to show that the conclusion could be false even though the premises are true.

So how is it possible that excessive atmospheric carbon could threaten human life even though nature has this regulatory mechanism? One possibility is that natural regulation has limits: if fossil fuels cause too much of a carbon excess, natural processes may not be able to fix the problem before it's too late, if at all. Or maybe fossil fuels interfere with natural regulation in some way. Whatever the correct answer choice targets, it will call into question the conclusion that environmentalists should just chill.