Which of the following materials is used primarily to protect steel from the heat of a fire?

Gypsum board is primarily used to protect steel from the heat of a fire due to its inherent fire-resistant properties. When exposed to high temperatures, gypsum board releases moisture, which helps cool the underlying steel structure. This material is non-combustible, meaning it does not ignite or contribute to the fire load, making it effective for providing a barrier against heat and preventing structural failure during a fire event.

In the context of fire resistance, the other materials do not serve the same protective purpose as gypsum board. Glass may break when exposed to high heat but does not provide insulation or fire protection for steel. Masonry offers some degree of fire resistance; however, it is typically more effective for structural elements rather than specific protection for steel. Plastic, on the other hand, is combustible and would not provide any protective benefits against fire, often contributing to the fire load instead. Therefore, gypsum board is the preferred choice for safeguarding steel from fire exposure.