Which of the following describes 'toughness' in the context of nails?

Toughness in the context of nails refers to their ability to withstand stress and strain without breaking or deforming. This quality is best described as a combination of strength and flexibility. When nails are tough, they can endure pressures, such as bending or twisting, without fractures while still maintaining their structural integrity. This balance between being strong enough to resist breaking and flexible enough to absorb impact is crucial for overall nail health and durability.

While other options address different characteristics of nails—like water resistance, thickness, and color retention—they do not encompass the concept of toughness. Strength alone does not address the ability to flex or bend, which is essential in determining how well a material can resist damage when subjected to stress. Thus, the correct answer highlights the holistic nature of toughness as it combines both aspects of strength and flexibility.