Final answer:
Full lifetime job security is not a characteristic of the U.S. capitalism model; this concept is less prevalent due to various economic changes.
Step-by-step explanation:
The feature that is NOT a characteristic of the U.S. model of capitalism in the twentieth and twenty-first centuries is d. full lifetime job security. The U.S. model includes features like open markets, large-scale operations, and a corporate model of ownership and organization. However, in today's capitalist economy, the concept of full lifetime job security is not guaranteed and is seen less frequently due to factors such as economic shifts, technological changes, and globalization impacting job markets.