Which of the following denotes evidence of existing structural instability?

Prolonged fire exposure is a clear indicator of existing structural instability because it can drastically weaken the integrity of building materials. When a fire occurs, the heat can lead to the degradation of structural components like steel and wood, causing them to lose their strength and stability. For instance, steel can lose up to 50% of its load-bearing capacity at elevated temperatures, and wood can become charred and brittle. After prolonged exposure, the structural components may be compromised and could fail under loads they would normally be able to support. This instability poses significant risks during firefighting operations and requires careful assessment to ensure the safety of both firefighters and occupants.

The other options, while they may relate to the overall condition or safety of a building, do not directly indicate structural instability. New construction materials suggest improvements or modernization, reinforced walls indicate an enhancement of strength, and regular inspections imply ongoing maintenance and safety checks that assess and promote stability rather than indicate a loss of it.