What business practice contributed most to andrew carnegie’s ability to form a monopoly?
a) using profits to support charities in order to greatly improving his reputation
b. increasing his profits every year
c. focusing on a single aspect of steel production
d. combining his companies into one company, and controlling all aspects of steel production

I believe the answer is: d.combining his companies into one company, and controlling all aspects of steel production

Andrew Carnegie believe controlling all aspects of steel production would help him bring the cost of the production down, which is why he buy out every smaller companies who involved in any form of steel productions.

He later on combine all of these companies and absorb them into the Carnegie steel company and fully controlled all transaction on the steel market at its peak.