Which of the following server types can be configured and joined to the existing network as a read-only server for user account verification to external websites?

A syslog server collects and stores log messages from network devices and systems; it has no function related to user account storage or authentication.

A web server delivers HTTP/HTTPS content to clients and does not maintain user account directories for external authentication purposes.

A directory server - such as an LDAP server or a Read-Only Domain Controller (RODC) in Active Directory - is specifically designed to store and serve user account data for authentication and authorization. It can be configured as read-only so that perimeter or external-facing systems can verify user credentials without having write access to the authoritative directory, which matches the scenario described.

A messaging server handles the routing and storage of email or instant messages and is not designed to serve as a user account directory for authentication.