Final answer:
The quote from the National Labor Union section stating that members saw the eight-hour workday as a panacea for labor issues best supports the plea for an eight-hour workday in Part A.
Step-by-step explanation:
The plea for an eight-hour workday was a significant demand of the labor movement during the 19th century. Among the provided references, the most supportive quote for the plea for eight hours comes from the National Labor Union (NLU) segment.
It states that the members of NLU, among a large faction of laborers, held the belief that an eight-hour workday was a panacea for various problems and hoped that it would improve their quality of life and lead to more jobs. This aligns with the labor demands for reduced working hours to ensure better conditions and necessitate more hiring, which is the crux of the plea for eight hours.