Final answer:
Profiles in RACF are responsible for retaining information about users, resources, and access authorities, making them key to managing security on IBM's z/OS operating system.
Step-by-step explanation:
The RACF (Resource Access Control Facility) structure that retains information about users, resources, and access authorities is C. Profiles. Profiles in RACF are central to its function as a security tool within IBM's z/OS operating system. They include user profiles, which contain information about individual user IDs and their attributes; group profiles, which manage collections of user IDs; and resource profiles, which contain information about protected resources and the access authorities that control who can use them. These profiles are crucial for managing and securing access to resources on mainframe computers.