Final answer:
Immigration is a key factor that contributes to the increasing racial and ethnic diversity in the U.S. workforce, alongside the career advancement of women who began working in the 1970s and 1980s.
Step-by-step explanation:
Among the options provided, immigration is likely a contributing factor to the increase of racial and ethnic diversity in the workforce. The United States, known for being a land of opportunity, has long been a destination for workers of all skill levels from around the world. Despite a dip during the economic slowdown of 2008, immigrants, both documented and undocumented, continue to play a significant role in the U.S. workforce. This trend is expected to contribute to the projected changes in America's racial and ethnic diversity by 2060, with a noticeable increase in Hispanic population. Additionally, as women who entered the workforce in the 1970s and 1980s are advancing in their careers, they are contributing to the diversity of the workforce as well.