Silica is added to roasted copper ores during extraction in order to remove :

Silica is added to roasted copper ores during extraction to remove ferrous oxide.

In the smelting process, silica is added to the copper sulphide and oxides to eliminate iron oxides. Sulfur dioxide and iron oxides, primarily in the form of $\mathrm{FeO}$,but also with some magnetite ${\mathrm{Fe}}_3{\mathrm{O}}_4$, are produced during roasting when the iron sulphides react with oxygen (in the air or oxygen-enriched air).

While copper sulphide and iron oxide can combine, a distinct slag layer forms when enough silica is introduced.

The process that creates slag is:

$\mathrm{FeO}+{\mathrm{SiO}}_2\to \mathrm{FeO}\cdot {\mathrm{SiO}}_2$

Slag creates a layer that floats on top of the matte because it is less dense than the matte.

Hence, option B is correct.