Final answer:
Option B, indicating that songs were popularized by performers rather than songwriters, would most challenge the author's viewpoint on the significance of songwriters and their political content in the Great American Songbook.
Step-by-step explanation:
The question posed asks which statement, if true, would most challenge the author's views as presented in the passage regarding the content of the Great American Songbook and its songwriters. Considering the context provided, the passage highlights the use of music as a medium for political content and social change, paying particular attention to the political scenes of the 60s, the protest songs of Bob Dylan, and the influence of music on American society and politics. To directly challenge these views, option B, stating that 'Most of the songs featured in the Great American Songbook were popularized not by the songwriters, but by other singers,' is most pertinent, as it shifts the focus from the songwriters' intent to the performers, thus undermining the importance of the songwriters' viewpoints and the political content of the songs themselves.