ANS:C - saturation and cutoff regions

One of the most common uses for transistors in an electronic circuit is as simple switches. In short, a transistor conducts current across the collector-emitter path only when a voltage is applied to the base. When no base voltage is present, the switch is off. When the base voltage is present, the switch is on.

The areas of operation for a transistor switch are known as the Saturation Region and the Cut-off Region. And use the transistor as a switch by driving it back and forth between its "fully-OFF" (cut-off) and "fully-ON" (saturation) regions.