Which of the following commands will immediately end a UNIX shell session without saving the history?

unset HISTFILE prevents bash from saving history to a file on normal exit but does not immediately end the session.

rm HISTFILE attempts to delete a literal file named 'HISTFILE' in the current directory rather than the variable-referenced history file, so it fails or removes an unintended file.

kill -9 $$ sends the unblockable SIGKILL signal to the current shell's process ID (represented by $$). Because the process is forcibly terminated by the kernel before it can execute any cleanup routines, bash never gets the opportunity to flush the session's command history to the HISTFILE, leaving no trace of the session's commands.

rm $HOME/.bash_history deletes the existing history file but does not end the current session, and in-memory history for the active session may still be written on a clean exit.