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

as a catalyst

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

to decrease the rate of oxidation of carbon at the  anode

to decrease the rate of oxidation of carbon at the cathode