The correct answer is A. Interrupt.
An interrupt is a signal sent by a hardware device to the CPU to request immediate attention. The CPU will then stop what it is doing and handle the interrupt before continuing with its previous task.
A spool is a temporary storage area for data that is being sent to or from a peripheral device. This allows the CPU to continue processing other tasks while the data is being transferred.
A stack is a data structure that is used to store data that is being used by the CPU. The stack is LIFO (Last In, First Out), which means that the last piece of data that is added to the stack is the first piece of data that is removed.
A page file is a file on the hard drive that is used to store data that is not currently in use by the CPU. This allows the CPU to access data more quickly, as it does not have to wait for the data to be loaded from the hard drive.