Which of the following materials is a good heat insulator?

Cotton wool is indeed considered a good heat insulator due to its properties. It consists of a network of air-filled spaces, which significantly impede the transfer of heat. This makes it effective at reducing heat conduction. The trapped air within the cotton wool forms a barrier that prevents heat from escaping, making it an excellent choice for insulation.

In contrast, materials like copper wire and iron are good conductors of heat. They facilitate heat transfer, which is the opposite of what an insulator does. Glass has moderate insulating properties compared to metals, but it does not match the effectiveness of cotton wool in reducing heat transfer due to its structure and thermal conductivity. Thus, cotton wool stands out as the best insulator among the given choices.