Final answer:
The Gramm-Leach-Bliley Act of 1999, also known as the Financial Services Modernization Act, repealed parts of the Glass-Steagall Act, allowing companies to operate in the banking, insurance, and investment sectors simultaneously.
Step-by-step explanation:
The legislative act that eliminated the prohibition preventing banks, insurers, and investment firms from entering into one another's markets is the Gramm-Leach-Bliley Act (GLBA) of 1999, also known as the Financial Services Modernization Act of 1999. Prior to GLBA, the Glass-Steagall Banking Act of 1933 established the FDIC and enforced a separation between commercial banking, investment banking, and insurance. The Glass-Steagall Act's conflict of interest provisions, which prevented a single company from offering a combination of these services, were repealed by the GLBA, thus allowing companies like ABC Insurance Company the ability to purchase a bank and expand into banking operations.