Infinitives are used to convey purpose.
So RC questions stems that say
the author mentioned _____ in order
tothe author's reference to ____ serves
tothe author brought up ______ primarily
toare all Purpose questions. Why did the author mention the Fur Seal Treaty?
Most of the time, these questions are freebies. The author has just made a big claim and follows it up with a specific detail/illustration/example, such as "Fruit has always played an important role in my family. For my brother's 30th birthday, we each ate a personal sized watermelon instead of cake."
If a question said "the author mentioned eating a small watermelon in order to", the answer would be some paraphrase of the preceding BIGGER claim.
(A) to provide an illustration of the role fruit played within his family
So when I see these questions, I look for the bigger claim coming right before (or sometimes right after) the detail in question.
Here, the previous line 17-20 is a big claim about problems that arise in the legal application of "tradition".
Doing this pre-phrase, instantly makes me like (E) more than all the other answer choices, since it's the only one talking about the legal application of "tradition".
But how the Fur Seal Treaty plays a role in this whole passage is still kinda tricky.
Its function is telling us that between 1910 and 1972, Alaskan natives were NOT allowed to hunt sea otters.
The MMPA came along in 1972 and said that Alaskan natives COULD hunt sea otters if it was to make articles "commonly produced before 1972" ... "in living memory".
Well there's the glitch: Alaskan natives haven't been allowed to hunt otters and make these articles for the past 62 years! (since BEFORE 1910) So the only people with a "living memory" of making these articles had to be someone alive in 1909.
That's why the court ultimately sided with the Alaskan natives. They said "tradition" can't be defined as merely "living memory". Lines 49-53 are saying "the fact that Alaskan natives were prevented, by circumstances beyond their control..." (i.e. the Fur Seal Treaty) "... from exercising a tradition ... does not mean ... that it has become any less a 'tradition'."
So that's what (E) is talking about.
We talked about the Fur Seal Treaty because it imposed a 62 year ban on making otter pelts ... that's what made the FWS say making otter pelts was not "traditional / in living memory" in 1986. And when the court again gave Alaskan natives the right to resume hunting otters, it made reference to the Fur Seal Treaty.
So the Fur Seal Treaty played a big role in the evolution of the Natives' legal rights w/ respect to the definition of "traditional".
(A) brink of extinction? not mentioned
(B) the author's not concerned with discussing other animals other than the sea otters involved in these court cases / legal actions.
(C) when was the author trying to convince us that there is a "well-known" legal precedent for protecting hunted animals? The 1st P is about the prominent role of "tradition" in Alaskan law and the troubling ambiguity of the term "traditional". The 2nd P is about the definition of "traditional" involving a long-standing, continuous practice. So (C) is accusing the author of having, at this point in the passage, an agenda that involves trying to convince the audience that laws that prohibit the hunting of protected animals have a long legal history.
(C) is the tempting trap answer you get on all these Purpose questions that tries to tempt you with WHAT is mentioned in the sentence they're pointing us to.
In my previous importance of fruit/watermelon party example, (C) would have said "demonstrate that you don't need cake to have an enjoyable birthday party".
These questions almost always want us to connect to bigger nearby ideas, not reiterate what is said WITHIN the sentence.
(D) "Russian seal hunters"? Wtf? They're just grabbing a word from some random paragraph and making a sentence.

Hope this helps.