What should the solutions architect do to meet these requirements?

Explanations:

While AWS CloudFormation templates can be used to define infrastructure as code, deploying them across multiple Regions and accounts would require additional management and would not scale efficiently. IAM policies alone would not adequately handle multi-account deployments.

Although AWS Organizations can manage multiple accounts and AWS Control Tower can help with governance, this option does not directly address deploying CloudFormation templates across multiple Regions. It focuses more on organizational governance rather than on the deployment mechanism itself.

AWS Organizations allows the management of multiple AWS accounts, and AWS CloudFormation StackSets enable the deployment of CloudFormation templates across multiple accounts and Regions efficiently. This option meets the requirements for scalability and multi-account deployment.

Nested stacks are useful for organizing CloudFormation templates, but they do not facilitate deployment across multiple Regions or accounts. Changing the Region in nested stacks does not solve the issue of managing deployments in a multi-account, multi-Region architecture effectively.