Final answer:
The best tool to add a line into the HKLM registry hive is REGEDIT, which allows for direct editing of the system registry while the other listed tools serve different purposes.
Step-by-step explanation:
To add a line into the HKLM (HKEY_LOCAL_MACHINE) registry hive, the best tool to use is REGEDIT. REGEDIT, or Registry Editor, is a Windows utility that allows users to view and make changes to the system registry. It provides a graphical interface for accessing the registry's structure and editing its values, including adding new lines or keys.
MSCONFIG is a system configuration utility that can enable or disable startup programs and services but does not provide direct registry editing capabilities. RD is a command for removing directories, not for registry editing. REGSRV32 is used to register and unregister OLE controls, such as DLLs and ActiveX controls, in the registry, but it is not used for general registry editing.