"Reboot" is the correct answer. "Restart" is not a valid Kali Linux command and will return an error. Just typing "shutdown" in the terminal will schedule a shutdown. To shut down the computer instantly, you can use the "shutdown -h now" command.
The correct syntax for the shutdown command is "shutdown [OPTION] [TIME] [MESSAGE]" In many Linux distros the Sudo command is required, but in Kali, it is not necessary. "init 0" will power off the machine. However, you can use "init 6" to reboot the machine.
I hope this will clear your doubt.