The entry passwd: files ldap in the /etc/nsswitch.conf file ensures that the local authentication lookups consult the local /etc/passwd file (files) before attempting to use LDAP. This correct configuration optimizes user authentication by reducing reliance on network services when local credentials are available. Alternative configurations like passwd: ldap files or passwd: nis files suggest that LDAP or NIS (Network Information Service) would be queried before local files respectively, which is not the desired behavior based on the scenario provided. There is no authentication: files ldap directive in /etc/nsswitch.conf, making it an incorrect and misleading answer.