Which of the following describes brittleness?

Brittleness is best described as the tendency of a material to shatter when subjected to stress, particularly when hammered or otherwise forcefully impacted. This characteristic indicates that the material is not ductile, meaning it cannot easily deform or stretch before breaking. Instead, brittle materials will break or fracture suddenly rather than bending, which makes them less suitable for applications that require flexibility or resilience. For instance, glass and certain ceramics exhibit high brittleness, often breaking into many pieces upon impact.

Other options describe different properties. The ability to conduct electricity pertains to conductivity, which is not related to brittleness. The ability to change states under pressure suggests a property of a material's phase transitions and is not indicative of brittleness either. Lastly, the quality of being easily melted refers to a material's melting point, which also does not relate to its tendency to shatter under stress. Thus, the correct definition of brittleness aligns specifically with the tendency to shatter when hammered.