In Windows, a Security Identifier (SID) uniquely identifies which of the following?

• A. A computer
• B. A registry key
• C. An IP address
• D. A user or group

Answer: (D)

Security identifiers are the unique tokens assigned to security principals that can hold permissions in Windows. In practice, that means a SID uniquely identifies a user or a group. When permissions are set, Windows stores the SID in access control lists rather than the user’s name, so the actual identity remains correctly referenced even if the account’s display name changes. IP addresses or registry keys aren’t security principals and aren’t identified by SIDs. While a computer account can have a SID in certain contexts, the common and most consistent use of SIDs is to identify users or groups for access control and auditing.