The correct option is: "It reduces electrical conductivity and increases brittleness."

Concept:

Resistance, Resistivity, and Conductivity of a Conductor

• The resistance of a conductor increases with temperature.
• Resistance depends upon the resistivity and size of the conductor.
• Resistivity depends on temperature as ρ (T) = ρ₀ (1 + α (T- T₀))
• ρ is resistivity at some temperature, ρ₀ is resistivity at standard temperature T₀.
• Conductivity is the reciprocal of resistivity.

Semiconductors

• Semiconductors are devices that come between conductors and insulators.
• The resistance of a semiconductor decreases with an increase in temperature.
• As the temperature increases, some of the electrons acquire energy and become free for conduction. Hence, conductivity increases and resistance decreases with an increase in temperature.
• At zero temperature, the semiconductor behaves as a perfect insulator.
• The semiconductor has a negative temperature coefficient of resistivity; therefore since with the increase in the temperature, the resistance decreases.

Conclusion:

The correct option is Option 2: "It reduces electrical conductivity and increases brittleness."

This is because hardening a material such as copper or aluminium typically involves processes like cold working or alloying, which can increase dislocation density and impede the movement of electrons, thus reducing electrical conductivity. Additionally, these processes often lead to an increase in brittleness due to the introduction of defects and changes in the microstructure of the material.