Final answer:
The union membership of employed individuals in the United States has steadily declined until the mid-1990s and then increased considerably.
Step-by-step explanation:
The correct answer is d. steadily declined till the mid-1990s and then increased considerably.
The proportion of U.S. workers belonging to unions has indeed declined since the early 1950s. However, this decline was not constant. It steadily declined until the mid-1990s and then started to increase considerably. This shift in union membership can be attributed to various factors, such as changes in the economy, job growth in different sectors, and the legal environment affecting union organization.
For example, job growth shifted from manufacturing to service industries, where unions had less success in organizing workers. Additionally, changes in the legal environment made it relatively more difficult for unions to organize workers and expand their membership.