Modern furnace walls are usually lined with _______________.

Modern furnace walls are commonly lined with materials that can withstand high temperatures and provide insulation, which is essential for the efficient operation of furnaces. The correct option refers to the use of tile or water walls.

Tiles can be ceramic or other heat-resistant materials that provide excellent thermal insulation and can help to protect the structure of the furnace from extreme heat. Water walls, which consist of tubes filled with water, help absorb heat and manage temperatures within the furnace, optimizing its performance and safety.

The other options do not accurately describe the typical materials used in modern furnaces. While some furnaces may use steel for structural components, tiles or water walls are specifically designed for heat insulation and thermal management in high-temperature applications, making them the more appropriate choice.