Explanation

The blank must be filled with a word that characterizes Mark Twain’s comments on the “flaws and foibles” of his country; since pointing out flaws implies that the comments are critical, “laudatory,” meaning flattering, is not the correct choice. “Buoyant,” meaning lighthearted, would also be an unlikely adjective to apply to criticism, and both it and “temperate” suggest an inoffensiveness that would notexplain the heirs’ and editors’ fears that publishing the comments would damage Twain’s reputation. “Premonitory” suggests that Twain’s observations were predictive or that their purpose was to warn, something not supported by any other information in the sentence. “Acerbic,” however, implies a bitterness of tone that could very plausibly accompany criticism, and it also suggests why Twain’s heirs and editors were worried about the remarks’ effect on the writer’s reputation. Thus the correct answer is acerbic (Choice B).