In the late 1980’s, the population of sea otters in the North Pacific

Premises:
Population of sea otters is declining.
Two possible reasons: killer whales and disease.
Disease is more likely because disease is killing seals and sea lions and because killer whale population has remained steady.

The reasoning uses two prong logic to show why disease is more probable than killer whales:
Disease is already killing other animals so it could have spread to sea otters too.
The population of killer whales has remained steady.

We can weaken the conclusion by weakening either.

(A) Killer whales in the North Pacific usually prey on seals and sea lions but will, when this food source is scarce, seek out other prey.

This shows that even though the population of killer whales hasn't increased, they could be the reason for decreased sea otter population. Their usual prey - seals and sea lions - have dwindled because of disease. So they could be seeking out sea otters. Hence it weakens the argument by showing that even if the killer whale population has not increased, more of them could be relying on sea otters as food source.


(B) There is no indication that the sea otter population at any North Pacific location declined in the 1980s because of substantial numbers of sea otters migrating to other locations.

Note that the scope of our argument is limited - out of two plausible explanations, one is more likely than the other. We don't have to worry about any other explanation. It is either disease or killer whales. We need to compare the likelihood of these two only. Hence any option talking about any other reasoning is out of scope of the argument.

(C) Along the Pacific coast of North America in the 1980s, sea otters were absent from many locations where they had been relatively common in former times.

This doesn't strengthen/weaken either of the two explanations. It just says that sea otter population has declined in a way that they are absent from some location. What caused it, disease or killer whales, we don't know.


(D) Following the decline in the population of the sea otters, there was an increase in the population of sea urchins, which are sea otters' main food source.

Again, this has nothing to do with WHY sea otter population declined - the concern of our argument. What happened after sea otter population declined is out of scope.

(E) The North Pacific populations of seals and sea lions cover a wider geographic area than does the population of sea otters.

The geographical area covered by seals and sea lions doesn't affect our argument. We want to compare the two explanations - disease or killer whales. It doesn't affect either.
Note that a change in geographical area covered by prey of killer whales could have had some impact.

Answer (A)