The ability of a fastener material to resist abrasion and indentation is its:

The correct choice is hardness, as this property specifically refers to a material's ability to withstand abrasion and indentation. Hardness measures how resistant a material is to scratching, denting, and wear, which are critical factors in applications where fasteners are subjected to friction, pressure, or impacts. For example, harder materials tend to hold up better under conditions that would otherwise damage or deform softer materials.

Understanding the hardness of a fastener material is essential in selecting appropriate fasteners for different applications, particularly in industries where durability and wear resistance are paramount. In contrast, toughness relates to a material's ability to absorb energy and plastically deform without fracturing, flexibility refers to how easily a material can bend without breaking, and strength is the material's ability to withstand an applied force without failure. While these properties are important, they do not directly address the resistance to abrasion and indentation that hardness specifically covers.