Not necessary, because the author is not committed to the view that the black water has ever hit the bay before. The conclusion is that EVEN IF it has hit the bay before, it wasn’t of last year’s intensity in the last 200 years. That conclusion does not assert that the black water has hit the bay before.

Not necessary, because the reasoning is based on the effects of black water on five species of coral. We don’t know that these constitute “every species” of coral in the bay, nor does any part of the argument rest on the idea that those are all the species.

Not necessary, because whether other plants/animals have been hurt is irrelevant. The argument is based on what occurred to five species of coral in the bay and what we can infer based on the damage to those species.

Necessary, because if it were not true — if the mounds of centuries-old coral that were destroyed WERE in especially fragile condition just before the black water hit the bay last year — then that fragile condition could explain how the black water might have hit the bay with the same intensity in the past 200 years but still not have destroyed the coral mounds. In other words, the negation of (D) would show that the fact the coral mounds had been around over 200 years does not show that the black water has never hit at last year’s intensity in those 200 years. In previous years, the coral mounds might have been sturdier, allowing it to withstand destruction from previous black water phenomena.

Not necessary, even if older specimens were NOT more vulnerable than young specimens, we already know the black water wiped out five species of coral and destroys coral mounds, and the author can still draw reasonable inferences from the fact the coral mounds were over 200 years old. What age category of coral was more or less vulnerable doesn’t undermine the argument.