Based on the information provided, the best description of the author's tone would be D) frustrated. The comparison between "the white moderate" and "the Ku Klux Klanner" suggests a level of exasperation and disappointment with the actions or inaction of the white moderate. This frustration is likely rooted in the perceived failure of the white moderate to actively support the cause of justice and equality, despite their capacity to do so. While the other options (A. furious, B. disgusted, C. embarrassed) may also be applicable depending on the context and specific examples within the text, frustrated seems to be the most overarching tone based on the given information.