The correct answer is option 'C' - Glass-wool.

Explanation:
In order to understand why the temperature drop across the material will be maximum for glass-wool, let's consider the concept of thermal conductivity.

Thermal conductivity is a property of a material that determines how well it can conduct heat. Materials with high thermal conductivity can transfer heat more easily, while materials with low thermal conductivity transfer heat slowly.

1. Copper:
- Copper is a metal that has a very high thermal conductivity, which means it can conduct heat very well.
- When heat flows through copper, the temperature drop across the material will be relatively small because it can easily transfer heat from one side to the other.

2. Steel:
- Steel is also a metal, but it has a lower thermal conductivity compared to copper.
- When heat flows through steel, the temperature drop across the material will be higher than copper, but still relatively small compared to other materials.

3. Glass-wool:
- Glass-wool is an insulating material commonly used for thermal insulation purposes.
- It has a very low thermal conductivity, which means it does not conduct heat well.
- When heat flows through glass-wool, the temperature drop across the material will be higher compared to copper and steel.
- This is because glass-wool resists the transfer of heat, causing a larger temperature difference between the two sides of the material.

4. Refractory Brick:
- Refractory brick is a material designed to withstand high temperatures and is commonly used in furnaces and kilns.
- It has a low thermal conductivity, similar to glass-wool.
- When heat flows through refractory brick, the temperature drop across the material will also be higher compared to copper and steel.

Conclusion:
The temperature drop across a material is related to its thermal conductivity. Materials with low thermal conductivity, such as glass-wool and refractory brick, will have a higher temperature drop compared to materials with high thermal conductivity, such as copper and steel. Therefore, the temperature drop will be maximum for glass-wool (option 'C') in this scenario.