Final answer:
An ACL, or Access Control List, is a tool that provides individual access control to resources and defines user permissions. Hence, option A is the right answer.
Step-by-step explanation:
The term ACL, which stands for Access Control List, best describes a mechanism that provides individual access control to resources. ACLs are a fundamental part of computer and network security, used to define who can access specific resources and what operations they are permitted to perform.
These lists specify access rights for each user or user group to a particular system object, such as a file directory or individual file. They are not dynamic by nature; rather, they are updated as access requirements change. Contrary to option B, ACLs are indeed used in today's systems across various platforms and technologies.
They play a critical role in securing system and network resources. Regarding option D, ACLs themselves are used to authorize access, not to authenticate users; authentication is the process of verifying a user's identity before ACLs enforce their access level.