Which of the following properties primarily affects the toughness of metals?

The toughness of metals is significantly influenced by grain size and distribution. Toughness is the ability of a material to absorb energy and plastically deform without fracturing. Smaller grains in a metal structure typically enhance toughness because they provide more grain boundaries, which can impede the movement of dislocations. This impeding effect increases the material's resistance to crack propagation, thus enhancing toughness.

Additionally, a uniform distribution of grain sizes contributes to consistent mechanical properties throughout the material, preventing weaknesses that could lead to failure under stress. In contrast, larger grains can lead to reduced toughness as they allow for more significant deformation before fracture but may not have the same capacity to absorb energy as finer-grained structures.

While electrical conductivity, magnetic permeability, and chemical reactivity are important characteristics of materials, they do not primarily dictate toughness. They relate to different aspects of material behavior and properties, but they do not have a direct impact on how well a metal can withstand stress before it fails.