Nature constantly adjusts the atmospheric carbon level. An i

pdutra19's analysis is great! Since this question asks us about the argument of the passage, we should start by clearly defining what the argument of the passage is - this will help us not to get caught up in trap answer choices. Restating the last two sentences of the paragraph gives us: "Nature will adjust the carbon level such that it will not threaten human life." We want to weaken this argument.

A talks about plants. We're really only concerned about human life here. Eliminate.
B indicates that more carbon may not actually hurt human life. If anything, this strengthens the argument - even if the carbon level gets higher, human life won't be threatened. Eliminate.
C indicates that carbon is important for human life. Again, this could be taken to strengthen the argument - more carbon = good for human life. Eliminate.
D indicates that other sources of carbon are more significant than fossil fuels. While the paragraph does mention fossil fuels as something environmentalists are concerned about, they aren't particularly relevant to the argument itself: that nature will adjust the carbon levels, wherever they come from. Eliminate.
E tells us that nature adjusts the carbon levels over a long, long time. This means that the carbon levels can go way way up or way way down during the millions of years, but by the end of that time, it will have leveled out. This means that humans could be exposed to way higher levels of carbon for, say, hundreds of years. If this is the case, it doesn't really matter for human life whether carbon levels level off after millions of years - hundreds of years of much higher carbon levels WILL threaten human life. This weakens the argument that natures adjustment will keep human life from being threatened, so E is the correct answer.