Given the tenant's App registrations settings and following the principle of least privilege, which role should be assigned to User1 so they can register an application?

The "Application developer" role in Azure AD is the most appropriate choice because it grants permissions to create and manage application registrations, service principals, and application proxy settings, aligning with the principle of least privilege for registering an application. The "Cloud application administrator" role has broader permissions, including consent to permissions and managing all aspects of application registration, which is more than User1 needs. "App Configuration Data Owner" and "Managed Application Contributor" are Azure RBAC roles related to specific resource types within a subscription, not Azure AD roles for managing application registrations.