What type of material is often used to keep warm when its particles trap air?

Foam is the correct choice because it is specifically designed to trap air within its structure, creating pockets that act as insulators. The air trapped within the foam reduces heat transfer through conduction, making it an effective thermal insulator. This property allows foam to retain warmth by minimizing heat loss, which is particularly beneficial in applications such as insulation in buildings or in clothing. The structure of foam, with its multiple small air-filled spaces, effectively slows down the movement of heat, helping to maintain a stable temperature.

In contrast, the other materials listed, such as metals, ceramics, and glass, generally have much higher thermal conductivity and do not trap air in the same effective manner as foam, making them less suitable for insulation in terms of retaining heat.