(i) In Hall-Heroult process for extraction of $\mathrm{Al}$, carbon anode is oxidised to $\mathrm{CO}$ and ${\mathrm{CO}}_2$.

(ii) When ${\mathrm{Al}}_2{\mathrm{O}}_3$ is mixed with ${\mathrm{CaF}}_2$, it lowers the melting point of the mixture and brings conductivity.

(iii) ${\mathrm{Al}}^{3+}$ is reduced at cathode to form $\mathrm{Al}$.

(iv) ${\mathrm{Al}}_2{\mathrm{O}}_3$ serves as the electrolyte, undergoing the redox process. ${\mathrm{Na}}_3{\mathrm{AlF}}_6$ although is an electrolyte but serves as a solvent, not electrolyte.