What property is essential for a material to withstand weathering and mechanical impact?

The correct choice for the property essential for a material to withstand weathering and mechanical impact is fracture toughness. Fracture toughness is a measure of a material's ability to resist the propagation of cracks when subjected to stress. This characteristic is crucial for materials used in environments where they may be exposed to mechanical impacts or cyclic stresses, as well as harsh weather conditions. A high fracture toughness indicates that a material can absorb significant energy and deform without failing, making it suitable for applications where both physical impacts and environmental factors can cause failure.

While other properties like durability, elongation, and flexibility may contribute to a material's overall performance under certain conditions, they do not specifically address the phenomenon of crack propagation, which is critical in the context of both weathering and mechanical impacts. Durability speaks more to the overall lifespan and resistance to degradation of a material. Elongation, which refers to a material's ability to be stretched, and flexibility, which pertains to its ability to bend without breaking, do not directly correlate with the capacity to resist crack growth under load. Hence, fracture toughness stands out as the essential property in this scenario.