Operating System (OS)
The instruction register

None of these
is a hardware memory device which denotes the location of the current instruction being executed.
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
contains the address of the memory location that is to be read from or stored into.
contains a copy of the designated memory location specified by the MAR after a "read" or the new contents of the memory prior to a "write".