Slag is a product of:

Flux and impurities result in the creation of slag.

Flux and impurities produce slag as a byproduct. Slag is a waste product that is removed from metal during the smelting process as well as other metallurgical and combustion processes due to metal impurities. Slag that is eliminated during the smelting of copper and lead includes materials like iron and silica.

Hence, option C is correct.