In the context of the Hall-Heroult process for the extraction of Al...

(i) In Hall-Heroult process for extraction of $Al$, carbon anode is oxidised to $CO$ and ${CO}_{2}$.

(ii) When ${Al}_{2}{O}_{3}$ is mixed with ${CaF}_{2}$, it lowers the melting point of the mixture and brings conductivity.

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

(iv) ${Al}_{2}{O}_{3}$ serves as the electrolyte, undergoing the redox process. ${Na}_{3}{AlF}_{6}$ although is an electrolyte but serves as a solvent, not electrolyte.