Concept:The system unit is the main casing that holds internal hardware components, while external devices are not housed inside it.
Explanation:The motherboard is the main circuit board fixed inside the system unit.
The central processing unit (CPU) is mounted on the motherboard inside the system unit.
The floppy disk drive is also fitted within the system unit to read floppy disks.
A plotter is an external output device used for printing large drawings and designs on paper.
It connects to the computer from outside and is not placed inside the system unit.
Therefore, among the given options, the plotter is the component that is not housed in the system unit.
Answer:A. plotter