Explanation
To join a file server to an Active Directory domain, the smb.conf file must specify the realm and the workgroup parameters correctly. The realm parameter is the DNS name of the Active Directory domain, in uppercase letters. The workgroup parameter is the NetBIOS name of the domain, which is usually the first part of the DNS name, but can be different. For example, if the DNS name of the domain is intra.example.com, the NetBIOS name could be INTRA or something else. To find out the NetBIOS name of the domain, you can use the command net ads info on a Linux machine that has Samba installed, or the command nltest
/dsgetdc:intra.example.com on a Windows machine that is joined to the domain. The other options are either incorrect or irrelevant for this question.