Explanation:
The halt command is an emergency shutdown command, similar to init 0. halt should be usedwith care because it does not initiate the orderly shutdown of services or clean dismounting of file system. Therefore, they (halt and init 0) are not correct choices! Reboot is essentially the same as the init 6 command. So the system is shut down and reboot to the system default level defined in /etc/inittab. Poweroff command is functionally equivalent to the halt command, except that power is removed from the system after it is halted. Shutdown command is the recommend way to shut down the system. It first sends out warning messages to all log-in users; this is particularly useful with multi-user servers; and it will stop all services in an orderly way and unmount all file systems; then do a sync.

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