Which of the following materials is considered a thermal insulator?

The correct answer is wood, which is classified as a thermal insulator due to its cellular structure that contains air pockets, minimizing the transfer of heat. This property makes wood an effective insulating material, often used in construction to reduce heat loss or gain, thus enhancing energy conservation in buildings.

Other materials like glass and concrete exhibit varying degrees of thermal conductivity, typically allowing more heat transfer than wood. Metal, on the other hand, is known for its excellent thermal conduction properties and typically transfers heat very efficiently.