Operating System (OS)
The instruction register

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".
contains the address of the memory location that is to be read from or stored into.
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
None of these
is a hardware memory device which denotes the location of the current instruction being executed.