ANS:D - endurance limit.

Fatigue resistance of a material is measured by the endurance limit. The endurance limit, also known as the fatigue limit or fatigue strength, is the maximum stress level that a material can withstand for a specified number of cycles without failure under cyclic loading conditions. It represents the stress below which the material can endure an infinite number of cycles without fatigue failure. Unlike the ultimate tensile strength, which measures the maximum stress a material can withstand before failure under static loading conditions, the endurance limit focuses on the material's behavior under repeated cyclic loading. It is a critical property for components subjected to fluctuating loads, such as machinery parts, bridges, and aircraft structures. Materials with higher endurance limits exhibit greater resistance to fatigue failure and are therefore preferred for applications where cyclic loading is common.