Let's start by breaking down the stimulus into premises and conclusions. The first sentence is a premise stating that waters off the Pacific coast have warmed by about 4 degrees in the past 15 years. The second sentence introduces a claim made by some scientists (which we can guess will be countered by the climatologist); the claim by some scientists is that human-generated air pollution caused global warming, which in turn caused the aforementioned water temperature increase. The third sentence introduces both a conclusion and a premise, with the conclusion being that the scientists' claim is far from justified, and the premise being that there are natural cycles of temperature change.
With this in mind, we can prephrase the climatologist's main conclusion: the claim that the temperature increase is due to global warming is not justified.
Turning to answer choice (E), we can see that this does not match our prephrase and strays away from it. Answer choice (E) introduces a conditional relationship between warm waters being caused by temperature changes and warm waters not being caused by global warming. While each of these ideas individually are present in the stimulus, this specific form of logic is not and the answer choice as a whole does not address the climatologist's main conclusion. The climatologist does not argue that if warming was caused by natural changes then warming was not caused by pollution, rather, the climatologist argues that claiming the warming was caused by pollution is not fully justified because natural temperature changes happen. So, we can eliminate (E) because it is not the most accurate way of expressing the climatologist's argument.
Answer choice (C) summarizes the main conclusion by saying that claiming warming is a symptom of pollution is not justified - almost exactly like our prephrase! This answer perfectly encapsulates the conclusion itself: that the scientists' claim is far from justified (which is the beginning of sentence 3).
When answering these questions, it is helpful to keep in mind what the conclusion itself actually is, since your best answer choice will likely be an almost synonymous rephrasing of the conclusion. A lot of the answers on this question are appealing, but remembering that the author is not saying the scientists' claim is definitively wrong, just that it is not fully justified (and offers a counter-explanation as proof).
I hope this helps!