Which of the following materials is approved for use as combustion air ducts?

Galvanized steel is the correct choice for materials approved for use as combustion air ducts due to its durability and resistance to corrosion. In the context of gas appliances, combustion air ducts must withstand high temperatures and often harsh environments where moisture or other corrosive substances may be present. Galvanized steel, which is steel coated with a layer of zinc, provides good structural integrity and can effectively prevent rust and corrosion, making it suitable for safely transporting combustion air to gas-burning appliances.

Other materials like aluminum may not offer the same level of durability in high-temperature scenarios, while plastic could be too vulnerable to heat and potential degradation. Copper, while resistant to corrosion, is not typically used for combustion air ducts due to its higher cost and potential for heat conduction issues. Therefore, galvanized steel is a compliant and practical choice in building codes for ventilation systems associated with gas appliances.