Final answer:
During the Swing Era, female musicians were not only vocalists but also instrumentalists and bandleaders, playing various roles that contributed significantly to jazz.
Step-by-step explanation:
Among the provided statements about female musicians during the Swing Era, the one that is NOT true is c) They were predominantly vocalists. While it's accurate that female musicians faced gender discrimination and made significant contributions to jazz, they didn't solely dominate as vocalists; they were active as instrumentalists as well. The Swing Era saw many women taking on various roles including bandleaders, instrumentalists, and some did become prominent singers. However, emphasizing them as predominantly vocalists diminishes their diverse contributions to jazz.