Explanation: The passage discusses the treatment of aboriginal natives in South America by the Spanish in their silver mines. Recall that in the Spanish colonies, a caste system existed according to race and birthplace, which is closest to (D). Eliminate (B), as it is a reversal of what is stated in the passage. Eliminate (C) because the letter is written to the king, who is ostensibly worried about his subjects in the last sentence. Eliminate (A) because it is less directly supported in the passage than (D).