What business practice contributed most to Andrew Carnegie's ability to form a monopoly? using profits to support charities in order to greatly improving his reputation increasing his profits every year focusing on a single aspect of steel production combining his companies into one company, and controlling all aspects of steel production

The correct answer is D) combining his companies into one company, and controlling all aspects of steel production.

The business practice that contributed most to Andrew Carnegie's ability to form a monopoly was "combining his companies into one company, and controlling all aspects of steel production."

Carnegie was a smart man that knew when to make the correct financial decisions that favored its company. During the Depression of 1873, he made a bold move and decided to invest in steel. With the use of technology he efficient, the process of producing steel and he was buying other competitors to get a vertical growth that allowed him to form one of the most notorious monopolies in the United States, just like John Rockefeller did with his Standard Oil Company.