Why lime stone is added in the extraction of lead from galena?

lime stone is added in the extraction of lead from galena because It prevents the formation of ${\mathrm{PbSO}}_4$, It removes the impurity of silica as fusible slag and It converts lead silicate to lead oxide.

The following reaction will obtain,

$\begin{array}{l}\mathrm{CaO}+{\mathrm{SiO}}_2\to {\mathrm{CaSiO}}_3\left(\mathrm{Slag}\right)\\ \mathrm{PbO}+{\mathrm{SiO}}_2\to {\mathrm{PbSiO}}_3\end{array}$

Cao converts ${\mathrm{PbSiO}}_3$ to pbo,

${\mathrm{PbSiO}}_3+\mathrm{CaO}\to \mathrm{PbO}+{\mathrm{CaSiO}}_3$

and also stops ${\mathrm{PbSO}}_4$ from forming.

Hence, options A, B and C are correct