Which of the following is an indicator that a building may become unstable?

A sagging roof is indeed a strong indicator that a building may become unstable. When the structure over time is subjected to weight beyond its intended capacity—such as from accumulated snow, structural deterioration, or other factors—the roof may begin to sag. This deformation signifies that the integrity of the structure is compromised, and it could lead to a collapse if not addressed.

In contrast, strong exterior walls and a solid foundation are indicators of stability and structural support. The presence of fire alarms, while important for safety, does not directly relate to the structural integrity of a building. Therefore, the sign of sagging roofs directly reflects potential instability and is crucial for assessing the safety of a structure during emergency operations.