The correct answer is B. Polysilicon.
Polysilicon is a semiconductor material that is used as the gate material in nMOS devices. It is a thin film of silicon that is deposited on the surface of the substrate. The gate material is responsible for controlling the flow of current through the device.
Silicon is a semiconductor material that is used as the substrate in nMOS devices. It is a thin layer of silicon that is deposited on the surface of the wafer. The substrate is the foundation of the device and provides the support for the other layers.
Boron is a dopant that is used to create n-type regions in nMOS devices. It is a small amount of boron that is added to the silicon substrate. The boron atoms replace some of the silicon atoms in the crystal lattice. This creates a region of the substrate that has a higher electron concentration than the surrounding region.
Phosphorus is a dopant that is used to create p-type regions in nMOS devices. It is a small amount of phosphorus that is added to the silicon substrate. The phosphorus atoms replace some of the silicon atoms in the crystal lattice. This creates a region of the substrate that has a higher hole concentration than the surrounding region.
In conclusion, the correct answer is B. Polysilicon.