Identity is a feature that allows managing user authentication and access across different applications and systems. Identity can be either internal or external to Salesforce, depending on where the user credentials are stored and verified. To cover NTOs identity needs, a Solution Architect should recommend the following:
Define a user registration handler to support user provisioning and authentication via social services like Google and Facebook. A user registration handler is a class in Apex code that defines the logic for creating and updating user accounts based on information from an external identity provider. A user registration handler can support user provisioning and authentication via social services like Google and Facebook, by using OAuth 2.0 protocols and OpenID Connect standards to exchange user information and tokens between Salesforce and the social services.
Leverage Salesforce Identity as the identity provider to centralize authentication for both Experience Cloud and B2C Commerce in one place. Salesforce Identity is a product that allows using Salesforce as an identity provider for other applications and systems. Salesforce Identity can centralize authentication for both Experience Cloud and B2C Commerce in one place, by using single sign-on (SSO) protocols and standards to enable users to log in to both applications with the same credentials.
Option C is incorrect because leveraging B2C Commerce as the identity provider for both Storefront and the Service Portal is not possible or advisable. B2C Commerce does not support acting as an identity provider for other applications or systems, as it does not support SSO protocols or standards. Option D is incorrect because using Salesforce CDP, which automatically syncs profiles and authentication information across systems, is not a valid or available option. Salesforce CDP is a product that allows creating unified customer profiles from various data sources, but it does not sync or manage authentication information across systems. Reference:
https://help.salesforce.com/s/articleView?id=sf.identity_overview.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.identity_provider.htm&type=5
https://help.salesforce.com/s/articleView?id=sf.sso_about.htm&type=5
https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_interface_Auth_RegistrationHandler.htm