The flux used in the smelting of copper pyrites is

• The flux is a chemical compound, which acts as a cleaning agent or to remove impure materials.
• In metallurgy flux is added to Combine with gangue (impurity). During ore smelting to form slag that can be separated from molten metal.
• Smelting is a process in which ore is mixed with flux and heated at high temperature in a closed furnace.

For example: - Silica, powdered lime, Alumina.

Therefore, Flux used in Copper smelting is silica $\left({\mathrm{SiO}}_2\right)$.

Hence, option(B) is the answer.