The author presents evidence that the toxic contamination of fish in Eagle Bay could have grave effects on public health, not that it has had grave effects on the local fishing economy.

The author doesn't refer to any moral principle, nor does he present evidence that the particular moral principle in (B) is usually correct.

The author’s doesn’t present any evidence that opponents of the ban failed to weigh its negative effects against its positive ones. In fact, he doesn’t mention opponents of the ban at all. He just notes that there is “widespread concern” over the economic effect of the ban.

By citing water pollution levels and a recent study of toxin levels in the bay’s fish, the author presents evidence that failing to enact the ban would bring unacceptable public health risks.

The author never presents evidence that the ban would reduce toxin levels in the bay’s fish. He instead presents evidence to show that, because those toxin levels are so high, failing to enact the ban would risk public health.