The correct answer is option 'C', which states that the electrolytic reduction of alumina to aluminium by the Hall-Heroult process is carried out in the presence of cryolite, which forms a melt with a lower melting temperature.

Cryolite, with the chemical formula Na3AlF6, is a rare mineral that is primarily found in Greenland. It plays a crucial role in the Hall-Heroult process as it acts as a solvent and lowers the melting temperature of alumina (Al2O3).

Below is a detailed explanation of why cryolite is used in the Hall-Heroult process:

1. The Hall-Heroult Process:
The Hall-Heroult process is the primary method used for the industrial production of aluminium. It involves the electrolysis of alumina dissolved in a molten electrolyte to produce pure aluminium metal.

2. Role of Cryolite:
Cryolite is added to the alumina to lower its melting temperature. Pure alumina has a very high melting point of approximately 2050°C, which requires a significant amount of energy to melt. However, by adding cryolite, the melting temperature of the mixture is decreased to around 950-1000°C, making the process more energy-efficient and economical.

3. Formation of Alumina-Cryolite Melt:
When alumina is mixed with cryolite, it forms a eutectic mixture, which is a mixture with the lowest melting point possible for the given components. This eutectic mixture has a significantly lower melting temperature than pure alumina, allowing it to be molten at a more reasonable temperature range.

4. Conductivity and Solubility:
Cryolite is not only responsible for lowering the melting temperature but also plays a critical role in enhancing the conductivity of the electrolyte. The presence of cryolite increases the ionic conductivity of the molten electrolyte, allowing the flow of electric current more efficiently. Moreover, cryolite also acts as a solvent, dissolving alumina and increasing its solubility in the molten electrolyte.

5. Other Advantages:
Using cryolite in the Hall-Heroult process offers several advantages. It increases the efficiency of the process by reducing the energy required for melting alumina. Additionally, it helps to reduce the corrosion of the carbon anodes used in the electrolytic cell, prolonging their lifespan.

In conclusion, the Hall-Heroult process for the electrolytic reduction of alumina to aluminium is carried out in the presence of cryolite. Cryolite acts as a solvent, lowers the melting temperature of alumina, enhances conductivity, and increases the solubility of alumina in the molten electrolyte. This helps to make the process more energy-efficient and economically viable.