Operating System (OS)
The Memory Buffer Register (MBR)

is a hardware memory device which denotes the location of the current instruction being executed.
None of these
is a group of electrical circuits (hardware), that performs the intent of instructions fetched from memory.
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.

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Operating System (OS)
A critical region is

a program segment that often causes unexpected system crashes
one which is enclosed by a pair of P and V operations on semaphores
none is the above
a program segment that has not been proved bug-free
a program segment where shared resources are accessed

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