The size of northern fur seals provides a reliable indication of their

Smaller the avg size of seals, larger the population.
In the 800 yr period when seals were hunted for food by Native peoples, their avg size did not vary.

What can we say from this? That hunting did not affect their population much.
Since the size did not vary, it means the population did not vary either.
So (E) is correct.

(A) During the 800-year period studied, seal hunting practices did not vary substantially between different groups of Native peoples in North America.

nikitamaheshwari
Did different groups of Native peoples hunt differently? We don't know. Perhaps one group used tools different from the other. Perhaps one group hunted 10 seals a week and another hunted 20, we don't know. The point is, whether their practices were same or different, we don't know. All we know is that the overall impact of all groups together was not much on the seal population. We are not given data about different regions or times during the 800 year period. We only know that throughout the period, not much had changed in the avg size of seals and hence not much has changed in the population size.

(B) The body size of northern fur seals is not strongly correlated with the overall health of the seals.

Any correlation with the health of the seals is irrelevant.

(C) Before the 800-year period studied, the average body size of northern fur seals fluctuated dramatically.

Before 800 yr period is irrelevant.

(D) Native peoples in North America made an effort to limit their hunting of northern fur seals in order to prevent depletion of seal populations.

We cannot say that the native people made a conscious effort. They may or they may not have. What we do know is that the impact was that the population did not change much. Whether native people consciously tried to keep it consistent or whether their hunting was so limited that it anyway had no impact, we don't know.

Answer (E)