GMAT Sentence Correction (SC) | Re: Unlike the people whom settled the Western states after the

D is incorrect because

that of thepeople whom settled the Western states after the passage of the Homestead Act of 1862, the hope of the prospectors and adventurers who came to California during the gold rush was

Here, whom settled conveys the people are not the subject rather the object. So, meaning wise D conveys "Western States settled the people", which is incorrect.

E is much clearer and hence the correct answer.
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