Deploy AWS Directory Service and connect it to the on-premises directory, then use IAM Identity Center for cross-account sign-in and Create an organization in AWS Organizations with all features turned on, and create the new accounts under the organization are correct because they together satisfy centralized workforce authentication and a governed multi account structure.

AWS Directory Service connects the existing on premises directory to AWS and IAM Identity Center provides single sign on and cross account sign in so workforce users can authenticate with their corporate credentials. Directory Service supports patterns such as AD Connector or Managed Microsoft AD to avoid migrating users and Identity Center maps those identities to permission sets across accounts.

AWS Organizations with all features turned on establishes the multi account foundation and enables centralized governance. Organizations provide consolidated billing, service control policies, and organizational units so you can apply guardrails and automate account creation under a managed structure.

Configure AWS Transit Gateway to centralize connectivity between VPCs and accounts focuses on network connectivity and routing rather than directory federation and workforce authentication so it does not meet the authentication requirement.

Set up Amazon Cognito user pools and configure IAM Identity Center to trust Cognito is intended for application end users and custom user pools and is not the standard approach for federating a corporate directory for workforce access to AWS accounts.

Use AWS Control Tower to centrally manage accounts and enable IAM Identity Center can simplify account provisioning and baseline governance but it does not itself connect to an on premises directory. Directory federation via Directory Service and Identity Center is still required so this option alone is incomplete.

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