Final answer:
Union membership in the United States has fallen due to a shift from manufacturing to service industries, stronger workplace protection laws, and legal challenges impeding union organization.
Step-by-step explanation:
The correct answer to why union membership has fallen in the United States is c) Fallen; laws enacted making it more difficult to unionize. This downward trend is attributed to several factors, including the shift from manufacturing to service industries, intensified global competition, the presence of workplace protection laws that may reduce workers' incentive to unionize, and a U.S. legal environment that imposes more challenges for the organization and expansion of union membership. Previously, unions had stronger representation in manufacturing, but as the economy transitioned towards service-oriented jobs, union growth did not keep pace.
In addition, the legal restrictions in place make it more challenging to organize new members within the service sector where most job growth is occurring.