Happy to help. The correct answer is (C); let's take a look at both (B) and (C).
(B) says, "account for the stance of the Native American leadership"
The author refers to the increased awareness of civil rights in the 1940s and 1950s to account for the readjustment movement, not to account for the stance of Native American leadership. Thus, (B) is out.
(C) says, "help explain the impetus for the readjustment movement"
This is exactly why the author refers to the increased awareness of civil rights in the 1940s and 1950s. As seen in the passage, the increased awareness of civil rights is discussed as a cause for the readjustment movement. Thus, (C) is the correct answer choice.
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