ANS:D - only allowed to the operating system

Operating in one of two possible modes, or states: the user state, or the supervisor state.

The user state is the default (normal) state of operation, in which user programs are executed. The supervisor state is a special mode of operation to which the user has no access. When it is in the supervisor state, the processor and its actions are entirely controlled by the Operating System (OS). The supervisor state is entered only on special occasions not part of the regular processing work of the CPU, like start up, shut down, dealing with hardware or software errors etc. Those special occasions are called exceptions.