Refractory materials are substances that have a high melting point and are able to withstand extremely high temperatures without undergoing significant deformation or degradation. These materials are commonly used in applications where high heat resistance is required, such as in the construction of furnaces, kilns, and other high-temperature processing equipment. The correct answer for this question is option 'A' which is MgO.

1. Magnesium Oxide (MgO):
- Magnesium oxide, also known as magnesia, is a refractory material that exhibits excellent heat resistance.
- It has a high melting point of around 2800°C, making it suitable for use in high-temperature applications.
- MgO is commonly used as a lining material in furnaces, kilns, and other thermal processing equipment because of its ability to withstand extreme temperatures and chemical corrosion.
- It has good thermal conductivity and electrical insulation properties, making it suitable for applications that require both heat resistance and electrical insulation.
- Magnesium oxide is also chemically stable, meaning it does not react with most substances at high temperatures, further enhancing its suitability as a refractory material.

Other options mentioned in the question (LiCl, KF, and CaCl2) are not typically considered refractory materials because they do not possess the necessary high-temperature resistance required for refractory applications.

In summary, refractory materials are essential for applications where high heat resistance is needed. Magnesium oxide (MgO) is a prime example of a refractory material due to its high melting point, excellent thermal conductivity, electrical insulation properties, and chemical stability.