Final answer:
Carnegie and Rockefeller engaged in a bitter rivalry rather than merging their companies or cooperating in philanthropy. Carnegie sold his steel operations to J.P. Morgan, while Rockefeller expanded his oil empire aggressively.
Step-by-step explanation:
Andrew Carnegie and John D. Rockefeller were titans of American industry who indeed negotiated, but they did not merge their companies or establish a joint foundation for philanthropy. Instead, Andrew Carnegie sold his steel company to J.P. Morgan, who created the United States Steel Corporation, while John D. Rockefeller continued to expand his oil empire by practicing horizontal integration and later by forming holding companies to control his interests following anti-trust legislation. Thus, the most accurate answer to the question of what Carnegie and Rockefeller did in the end is they engaged in a bitter rivalry (option d). This rivalry is rooted in their different business practices and approaches to industry domination during America's industrial age. Carnegie focused primarily on steel production and later became a philanthropist, while Rockefeller used aggressive tactics to maintain control over the oil industry.