Which security mechanism uses a unique list embedded in the object to define access levels for subjects?

The correct choice pertains to the concept of a User Access Control List (ACL). ACLs are integral to security frameworks in that they outline which users (subjects) have permissions to access specific resources (objects) within a system. Each object has an embedded list that assigns access rights to various users, effectively determining what actions they can perform with that object.

This mechanism is designed to offer granular control over resource access, ensuring that only authorized users can perform certain actions, such as reading, writing, or executing a file. The presence of a unique list embedded in the object facilitates a clear and straightforward way to manage access based on the roles and permissions defined for specific users or groups.

Other security mechanisms listed serve different purposes, such as conditionally allowing access based on predefined rules (conditional access), enforcing system-wide rules for all users (mandatory access control), or securing data integrity through cryptographic methods (hashing). However, none of these options utilize the unique list format to define user-specific access levels to objects as effectively as a User ACL does.