Explanation:

Poly silicon, also known as polysilicon or polycrystalline silicon, is a material commonly used in the semiconductor industry. It is used as a major component in the fabrication of integrated circuits (ICs) and other electronic devices.

Doping impurities:
Doping is the process of intentionally introducing impurities into a semiconductor material to modify its electrical properties. In the case of poly silicon, doping impurities are used to reduce its resistivity. By adding impurities such as boron or phosphorus to the poly silicon, the number of free charge carriers (electrons or holes) in the material can be increased or decreased, depending on the type of impurity added.

Reduction of resistivity:
The resistivity of a material is a measure of its opposition to the flow of electric current. Lower resistivity means that the material is more conductive. By doping poly silicon with impurities, its resistivity can be significantly reduced. This is because the impurities introduce additional charge carriers into the material, increasing its conductivity.

Types of doping:
There are two types of doping that can be used to reduce the resistivity of poly silicon:

1. N-type doping: In this type of doping, impurities such as phosphorus or arsenic are added to the poly silicon. These impurities have more valence electrons than silicon, creating an excess of negative charge carriers (electrons) in the material. This increases the conductivity of poly silicon and reduces its resistivity.

2. P-type doping: In this type of doping, impurities such as boron or gallium are added to the poly silicon. These impurities have fewer valence electrons than silicon, creating a deficiency of negative charge carriers (electrons) in the material. This creates an excess of positive charge carriers (holes) in the material, increasing its conductivity and reducing its resistivity.

Conclusion:
Doping impurities are used to reduce the resistivity of poly silicon by increasing its conductivity. By adding impurities to the poly silicon, the number of free charge carriers in the material can be controlled, resulting in a significant reduction in resistivity. Therefore, option C, "Doping impurities," is the correct answer.