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@Halle, Idk if im allowed to help but as they stated before, for "required" assumption questions, you have to basically read the answer choice to see if it supports then read it again but negating it to see if it destroys. And if the negation destroys the argument/ conclusion of the passage, it's the answer. In this case, D says that "The mounds of centuries-old coral that were destroyed were not in especially fragile condition just before the black water swept in last year." This is supporting yes, so it COULD be the answer. But by saying "The mounds of centuries-old coral that were destroyed WERE in especially fragile condition just before the black water swept in last year." destroys the conclusion because if they were fragile then the level of the toxic water wasnt higher than previous years, and we're trying to support that it was higher in previous years. Therefore, by negating the answer choice and seeing that it completely destroys the conclusion, but supports the conclusion if left as is, we know that this is the correct answer. This sounds confusing just explaining it, but hopefully you got it.

@ceci, Great explanation! @Hallerae, As @Ceci astutely pointed out, this is a strengthen with a necessary premise question, so we're looking for an answer that—if negated—makes the argument fall apart. (D) is required by the argument because if its negation would destroy the argument. Since its negation makes the argument fall apart, then we know that this assumption is necessary for the argument. Hope this helps. Let us know if you have any other questions!