The authorization standard that is built to handle Application Programming Interface (API) access for Federated Identity Management (FIM) is Open Authentication (OAuth). OAuth is a standard protocol that enables the delegation of authorization to access resources or services from one party to another, without sharing the credentials. OAuth can be used for FIM, which is a mechanism that allows the users to use a single identity across multiple domains or systems, such as social media platforms, cloud services, or web applications. OAuth can handle API access for FIM, which means that the users can authorize the applications to access their data or services from other providers, such as contacts, calendars, or photos, through the APIs.
Security Assertion Markup Language (SAML), Remote Authentication Dial-in User Service (RADIUS), and Terminal Access Control Access Control System Plus (TACACS+) are not authorization standards that are built to handle API access for FIM, but they are standards or protocols that can be used or supported by FIM for authentication, authorization, or accounting purposes. References: CISSP All-in-One Exam Guide, Eighth Edition, Chapter 5: Security Engineering, page 656; Official (ISC)2 Guide to the CISSP CBK, Fifth Edition, Chapter 3: Security Architecture and Engineering, page 439.