ANS:A - $!

$0 The filename of the current script.

$n These variables correspond to the arguments with which a script was invoked. Here n is a positive decimal number corresponding to the position of an argument (the first argument is $1, the second argument is $2, and so on).

$# The number of arguments supplied to a script.

$* All the arguments are double quoted. If a script receives two arguments, $* is equivalent to $1 $2.

$@ All the arguments are individually double quoted. If a script receives two arguments, $@ is equivalent to $1 $2.

$? The exit status of the last command executed.

$$ The process number of the current shell. For shell scripts, this is the process ID under which they are executing.

$! The process number of the last background command.