|A) Install the Directory Service Client on all of the Windows 98 computers.||B) Upgrade all of the Windows 98 computers to Windows 2000 Professional|
|C) Upgrade all of the Windows NT Workstation 4.0 computers and Windows 98 computers to Windows 2000 Professional||D) Nothing. Windows 2000 will automatically detect all client computers and optimize them to function with Active Directory.|
To achieve optimum functionality of the client computers, you must upgrade all of them to Windows 2000. Upgrading some of them and/or installing the Directory Service Client will gain some functionality, but the question specifically states that "optimum functionality" is the required result.