A domain controller is:

• A. A router that filters traffic
• B. A server that stores the Active Directory database and authenticates users on login
• C. A database management system for applications
• D. A workstation used by admins

Answer: (B)

A domain controller is a server that stores the Active Directory directory and handles user and computer authentication for a Windows domain. It runs the AD DS role, keeping the directory database with accounts, groups, computers, and policy settings, and it authenticates logins (issuing Kerberos tickets and enforcing access policies). It often integrates DNS to support name resolution for AD components.

This isn’t simply a router that filters traffic, nor a general-purpose database management system for applications, nor just a workstation used by admins. Those roles describe other devices or systems, while a domain controller specifically serves as the central authority for identity and access within the domain.