Which act required more uniform laws for agents and brokers operating across state lines?

The Graham-Leach-Bliley Act is the correct choice because it aimed to modernize the financial services industry by allowing affiliations between banks, securities companies, and insurance companies. One of its significant impacts was to provide a framework for more consistent regulation of insurance agents and brokers operating across state lines. This act encouraged the formation of a more uniform regulatory structure, making it easier for these professionals to conduct business in multiple states without facing a patchwork of differing regulations.

In contrast, the McCarran-Ferguson Act primarily relates to the regulation of the insurance industry and grants states the authority to regulate insurance practices. The Glass-Steagall Act historically separated commercial banking from securities and insurance businesses but does not address the uniformity of laws for agents and brokers specifically. The National Insurance Act pertains to the establishment of a national insurance framework, but it does not focus extensively on the operational uniformity for agents and brokers across state lines. Thus, the Graham-Leach-Bliley Act is the most relevant legislation for promoting a uniform regulatory environment in this context.