The function of fluorspar in the electrolytic reduction of alumina dissolved in fused cryolite $\left({\mathrm{Na}}_3{\mathrm{AlF}}_6\right)$ is:

to decrease the rate of oxidation of carbon at the  anode

to lower the temperature of the melt and improve the conductivity of cell

as a catalyst

to decrease the rate of oxidation of carbon at the cathode