Refer to the exhibit. Which type of authentication is in use?

Given that LDAP authentication for Microsoft Outlook is the correct answer, it implies the (unseen) exhibit details a configuration where Outlook clients are authenticating against an LDAP directory service (such as Active Directory) for accessing mailbox details, global address lists, or other user information. LDAP is a standard protocol for accessing and maintaining distributed directory information services.

POP3 authentication is specific to retrieving emails and does not encompass the broader directory service interactions that LDAP provides for a client like Outlook.

SMTP relay server authentication is used for sending emails via a relay and is distinct from the primary authentication method Outlook might use for accessing directory services.

'External user and relay mail authentication' is a generic phrase that lacks the specificity of LDAP as a protocol and Outlook as a client, which the correct answer identifies.