Incorrect. Nothing in the argument presupposes that there has been a longer history of decline in the seal population.

Incorrect. This could be true, but isn't required by the argument. If we negate this to state that additional sources of pollution have been added since 1988, that would still be perfectly consistent with the author's hypothesis that it is pollution in the North Sea in general, not only in May 1988 and prior, that has caused the seals to be so vulnerable to this virus.

Correct. The argument necessarily rules out any explanation that is not North Sea pollution, including mutations in the virus that make it more deadly. If we negate this answer choice to state that there had been a mutation in the virus by May 1988 that made it more deadly, that would severely undermine the author's argument that North Sea pollution is "clearly" the reason the seals are so vulnerable.

Irrelevant. We don't know anything about what's going on with the seal populations in these other seas. For all we know, seals in these other places with similar pollution levels have also been dying off since the late 1980s.

Incorrect. This could be true — it seems plausible that if the pollution affects the seals, it would also affect some of their food species — but it isn't required by the argument. Notice that if you negate this answer choice, the argument remains unaffected.