If the organization runs Active Directory, that means it has a Windows Server that is acting as a domain controller. These use the Kerberos authentication system by default. TACACS+ is an example of a remote authentication system, but is owned by Cisco, and is not a part of Active Directory. LDAP is the protocol in Windows that controls Active Directory objects, and works in conjunction with Kerberos, but is not the actual authentication method used. 802.1X is an authentication method used by network adapters on the data link layer.