A company needs a portal for end users to access assigned AWS accounts and cloud applications, and to manage access for third-party SaaS apps. Which AWS service fulfills this requirement?

AWS IAM Identity Center (AWS Single Sign-On) is the correct choice because it provides a centralized portal for users to access multiple AWS accounts and cloud applications with a single set of credentials. It also simplifies the management of user access to third-party SaaS applications. Amazon Cognito focuses on user authentication and authorization for web and mobile applications, not centralized access to AWS accounts or SaaS apps. AWS Identity and Access Management (IAM) manages permissions within a single AWS account and doesn't provide a consolidated user portal across multiple accounts or for SaaS applications. AWS Directory Service for Microsoft Active Directory integrates AWS with existing on-premises Active Directory, primarily for directory-aware workloads, not for a general user portal for AWS accounts and SaaS apps.