While all the factors listed contributed to the increase of women in the workforce during the twentieth century, the factor that likely had the least effect is:
C) the growth of white-collar jobs over blue-collar jobs
The growth of white-collar jobs did provide more opportunities for women, but the significant changes in workforce composition for women were largely driven by factors such as anti-discrimination laws (B), economic shifts (such as the need for dual-income families and the effects of World War II), and the GI Bill (D) that enabled more people, including women, to access education and develop careers. In comparison, the distinction between white-collar and blue-collar jobs is less directly tied to the surge of women in the workforce.