The correct answer is D - Open Authorization (OAuth).
According to the WGU Cybersecurity Architecture and Engineering (KFO1 / D488) Study Guide, OAuth is the standard protocol used for authorizing access to third-party applications without revealing user credentials.
It allows users to log in using social identity providers like Google, Facebook, or LinkedIn, which is perfect for external customers and contractors accessing a mobile application. OAuth is designed for modern applications requiring delegated access.
SAML (A) is generally used for enterprise single sign-on (SSO) solutions, primarily for internal enterprise authentication, not social login. Kerberos (B) is used within controlled internal network environments for authentication. LDAP (C) is a directory access protocol, not an authorization protocol for third-party sign-in.
Reference Extract from Study Guide:
"OAuth enables users to grant a third-party application limited access to their resources withoutexposing their credentials, making it ideal for mobile and web applications involving external users."
- WGU Cybersecurity Architecture and Engineering (KFO1 / D488), Authentication and Authorization Concepts