What is defined as the actual percentage of elongation that occurs when a material is stressed?

The correct choice is strain, which is defined as the measure of deformation representing the displacement between particles in a material body. When a material is subjected to stress, it undergoes a change in shape or size, and the actual percentage of elongation that occurs due to this stress is termed strain. Strain is quantified as the ratio of the change in length to the original length of the material, expressed as a percentage. This concept is critical in understanding how materials respond under different load conditions, particularly in construction and fire service scenarios where structural integrity is paramount.

Understanding strain helps in evaluating how much a material will deform before reaching its breaking point, which is essential for ensuring safety in building constructions and for planning effective fire suppression strategies. The other terms provided do not specifically refer to this percentage of elongation under stress, which is why they do not accurately describe the phenomenon in question.