Which of the following materials is a good heat insulator?

Cotton wool is considered a good heat insulator due to its structure, which traps air in its fibers. Air is a poor conductor of heat, so when it is trapped in a material like cotton wool, it effectively reduces the transfer of heat. This quality makes cotton wool an excellent choice for insulation purposes, such as in clothing or building materials, where minimizing heat transfer is essential.

Materials like copper wire and iron are good conductors of heat, meaning they readily transfer thermal energy. Glass has better insulating properties than metals, but it is still not as effective as cotton wool because it can still allow some heat transfer compared to materials specifically designed for insulation, like cotton wool. Thus, cotton wool stands out as the best option for heat insulation among the given choices.