The last sentence isn't a conclusion at all. The only conclusion in the argument is the first sentence, the prediction that heavy downpours are likely to become more frequent if Earth's atmosphere warms significantly. The last sentence is a premise that helps support that prediction. It's one link in the causal chain, not the point the meteorologist is trying to prove.

This has two problems. First, the last sentence isn't the conclusion of the argument as a whole. That's the first sentence. Second, (B) says there are other explicitly stated conclusions in the argument besides the main one. But there's only one conclusion here. None of the last three sentences function as intermediate conclusions, because none of them receive support from any other sentence.

This says the last sentence is something the argument is intended to support, which would make it some kind of conclusion. But it's not. No other premise in the stimulus is offered to prove that larger clouds lead to heavier downpours.

You might be drawn to (C) because the last sentence has obvious connections to the other premises. After all, it's one link in a causal chain, and the previous link ("warmer atmosphere produces larger clouds") feeds into it. But having a connection to another premise is not the same as being supported by that premise. "A warmer atmosphere creates larger clouds" tells us where larger clouds come from. It doesn't help prove that larger clouds lead to heavier rain. That's a separate factual claim the meteorologist simply asserts. So the last sentence remains a premise, not a conclusion.

This accurately describes the role of the last sentence. The argument has only one conclusion: the prediction in the first sentence. The last sentence is one of three premises that support it. Specifically, it's the link in the causal chain that connects larger clouds to heavier downpours.

The first half of (E) is reasonable. The last sentence does provide a causal explanation that relates to what the conclusion describes. But the second half is wrong. The last sentence absolutely is intended to support the conclusion. It's one of the premises in the causal chain the meteorologist builds to justify her prediction. If the last sentence weren't meant as support, it would just be a random observation with no purpose in the argument.