What property of a building material may lead to a reduction in its strength at elevated temperatures?

The correct answer pertains to combustibility, which refers to the ability of a material to catch fire and burn. When materials are exposed to elevated temperatures, especially in the context of a fire, their structure can begin to break down due to the heat. This breakdown occurs because the chemical bonds within the material can weaken or fail at high temperatures, reducing its overall strength. For example, materials like wood and certain plastics can ignite and lose their structural integrity when exposed to flames, leading to collapse scenarios during a fire.

In contrast, thermal expansion relates to how materials change size or shape when heated, which does not inherently reduce their structural strength. Density refers to the mass per unit volume of a material; while it can influence how materials react in a fire, it is not directly responsible for strength reduction at high temperatures. Density reduction is a phenomenon that may happen as a result of the heat causing material to lose moisture or other components, but this is not the primary concern in the context of strength under fire conditions. Thus, understanding combustibility is crucial for fire safety and structural integrity in firefighting scenarios.