Final answer:
The richest men in America began working together due to their mutual interests.
Step-by-step explanation:
The answer to the question "What finally caused America's richest men to work WITH each other instead of against one another?" is mutual interests (C).
In the late 19th and early 20th centuries, wealthy businessmen in America began to realize that working together could be more beneficial than competing against each other. They formed associations and organizations with the goal of promoting their common interests, such as protecting their wealth and influencing government policies.
For example, during the Progressive Era in the early 1900s, businessmen created organizations like the National Association of Manufacturers and the Chamber of Commerce to advocate for their own interests and shape government policies that would benefit them. These associations allowed America's richest men to collaborate and work together towards their mutual goals.