What command would you use to see a list of available runlevels?

The command that can be used to see a list of available runlevels is indeed "runlevel." This command provides information about the current runlevel of the system along with the previous runlevel. Runlevels are a historic way of defining different states of service and are primarily used in SysV init systems to manage system states.

In traditional Linux systems, runlevels define which services are started or stopped. Each runlevel corresponds to a specific state of the system, such as multi-user mode, single-user mode, or graphical mode. By executing the "runlevel" command, users can quickly identify the current operational state and gain insight into what system services might be active.

The other options relate to different functionalities and do not serve the purpose of listing available runlevels. For example, using "systemctl --list-units" is geared towards displaying active systemd units and their states rather than focusing on traditional runlevels. The command "chkconfig" is used to manage system services at different runlevels but does not explicitly display the runlevels themselves. Additionally, "getty" refers to a program that manages terminal sessions and is not related to querying runlevels. Thus, "runlevel" is the appropriate choice for this specific inquiry.